You have raised two broad questions. The question about reasonable suspicion asks: when is evidence illegally obtained? That's difficult to answer, because it depends on the nature of the evidence and any statute which controlled the way in which it should have been collected. However, the focus of your question seems to be the second issue: what use can the government make of illegally obtained evidence? This is the subject of the exclusionary rule. The short answer is this:
The exclusionary rule is a judicial remedy created for the purpose of deterring future unlawful conduct. The rule prohibits both direct and indirect use of illegally obtained evidence ("fruit of the poisonous tree") in a criminal prosecution, but will only be applied where its deterrence benefits outweigh its substantial social costs.
To give more insight into how the courts have applied this balancing test, I will summarise its history and rationale. I will then set out the modern formulation of the rule, and try to answer some of your specific hypothetical questions.
History and rationale of the exclusionary rule
The exclusionary rule originated in Weeks v. United States, 232 U.S. 383 (1914). The government searched Weeks' house without a warrant, seized letters and other property, and charged him with operating an illegal lottery. Weeks applied for the return of the property, but the district court held that "the letters having come into the control of the court, it would not inquire into the manner in which they were obtained, but if competent would keep them and permit their use in evidence." Weeks was convicted. The Supreme Court reversed, holding that:
If letters and private documents can thus be seized and held and used in evidence against a citizen accused of an offense, the protection of the Fourth Amendment declaring his right to be secure against such searches and seizures is of no value, and, so far as those thus placed are concerned, might as well be stricken from the Constitution ... The tendency of those who execute the criminal laws of the country to obtain conviction by means of unlawful seizures and enforced confessions, the latter often obtained after subjecting accused persons to unwarranted practices destructive of rights secured by the Federal Constitution, should find no sanction in the judgments of the courts which are charged at all times with the support of the Constitution and to which people of all conditions have a right to appeal for the maintenance of such fundamental rights.
In Silverthorne Lumber Co. v. United States, 251 U.S. 385 (1920), the doctrine was extended to prevent indirect use of information derived from illegally obtained evidence, unless the information comes from an independent source. The government illegally searched Silverthorne's offices, and copied the records seized before the district court ordered their return. The district court also impounded the copies, so the government issued a regular subpoena to produce the original documents. The district court held Silverthorne in contempt for failure to comply with the subpoena. The Supreme Court reversed, holding that:
The essence of a provision forbidding the acquisition of evidence in a certain way is that not merely evidence so acquired shall not be used before the Court but that it shall not be used at all. Of course this does not mean that the facts thus obtained become sacred and inaccessible. If knowledge of them is gained from an independent source they may be proved like any others, but the knowledge gained by the Government's own wrong cannot be used by it in the way proposed.
The Court acknowledged the 'complexities' of this distinction in Nardone v. United States, 308 U.S. 338 (1939). This case introduced the term 'fruit of the poisonous tree,' and an exception to the doctrine where the connection between the illegality and the evidence presented is 'so attenuated as to dissipate the taint.' Nardone had been convicted of defrauding the revenue twice. The first conviction was based on evidence obtained from an illegal wiretap, and was reversed by the Supreme Court. Nardone was convicted again after a retrial, and argued that the conviction should be set aside because he was not permitted to "examine the prosecution as to the uses to which it had put the [illegally obtained] information." Frankfurter J, delivering the opinion of the Court, quoted the above passage from Silverthorne and said:
In practice this generalized statement may conceal concrete complexities. Sophisticated argument may prove a causal connection between information obtained through illicit wire-tapping and the Government's proof. As a matter of good sense, however, such connection may have become so attenuated as to dissipate the taint ... The burden is, of course, on the accused in the first instance to prove to the trial court's satisfaction that wire-tapping was unlawfully employed. Once that is established—as was plainly done here—the trial judge must give opportunity, however closely confined, to the accused to prove that a substantial portion of the case against him was a fruit of the poisonous tree. This leaves ample opportunity to the Government to convince the trial court that its proof had an independent origin.
Development of modern limits to the exclusionary rule
The Court clarified the purpose of the doctrine in United States v. Calandra, 414 U. S. 338 (1974), declining to extend it to grand jury proceedings. The Court held that Calandra was required to answer questions put to him by a grand jury, even though the questions had been informed by the fruits of an illegal search. The Court said:
The purpose of the exclusionary rule is not to redress the injury to the privacy of the search victim ... Instead, the rule's prime purpose is to deter future unlawful police conduct and thereby effectuate the guarantee of the Fourth Amendment against unreasonable searches and seizures ... In sum, the rule is a judicially created remedy designed to safeguard Fourth Amendment rights generally through its deterrent effect, rather than a personal
constitutional right of the party aggrieved.
Despite its broad deterrent purpose, the exclusionary rule has never been interpreted to proscribe the use of illegally seized evidence in all proceedings or against all persons ... the application of the rule has been restricted to those areas where its remedial objectives are thought most efficaciously served.
The Supreme Court endorsed a good faith exception to the exclusionary rule in United States v. Leon, 468 U.S. 897 (1984). Drugs were found in a search of Leon's house and car pursuant to a facially valid search warrant, but on review the district court found that there was no probable cause to issue the warrant. Therefore, the search was illegal although the officers executing it had acted in good faith. After reviewing the cases in which the Court had declined to apply the rule, the Court held that the evidence against Leon should not have been excluded:
The substantial social costs exacted by the exclusionary rule for the vindication of Fourth Amendment rights have long been a source of concern ... We have now reexamined the purposes of the exclusionary rule and the propriety of its application in cases where officers have relied on a subsequently invalidated search warrant. Our conclusion is that the rule's purposes will only rarely be served by applying it in such circumstances. In the absence of an allegation that the magistrate abandoned his detached and neutral role, suppression is appropriate only if the officers were dishonest or reckless in
preparing their affidavit or could not have harbored an objectively reasonable belief in the existence of probable cause.
In Pennsylvania Bd. of Probation and Parole v. Scott, 524 U.S. 357 (1998), the Court cited Leon and explicitly endorsed the use of a balancing test in declining to extend the rule to State parole proceedings:
[B]ecause the rule is prudential rather than constitutionally mandated, we have held it to be applicable only where its deterrence benefits outweigh its
"substantial social costs" ...
A federal requirement that parole boards apply the exclusionary rule ... would severely disrupt the traditionally informal, administrative process of parole
revocation. The marginal deterrence of unreasonable searches and seizures is insufficient to justify such an intrusion. We therefore hold that parole boards are not required by federal law to exclude evidence obtained in violation of
the Fourth Amendment.
The Court endorsed an even more circumspect approach to the rule in Hudson v. Michigan, 547 U.S. 586 (2006). The police executed a valid search warrant and found guns and drugs in Hudson's house, but the search was unlawful because the police did not knock and announce before entering. The Court declined to exclude the evidence obtained in the search, holding that:
Suppression of evidence, however, has always been our last resort, not our first impulse. The exclusionary rule generates "substantial social costs", which sometimes include setting the guilty free and the dangerous at large. We have therefore been "cautio[us] against expanding" it, and "have repeatedly emphasized
that the rule's 'costly toll' upon truth-seeking and law enforcement objectives presents a high obstacle for those urging [its] application." We have rejected
"[i]ndiscriminate application" of the rule, and have held it to be applicable only "where its remedial objectives are thought most efficaciously served,"—that is, "where its deterrence benefits outweigh its 'substantial social costs.'" (citations omitted)
Current state of the law
As of June 2017, the last word on the rule is the Supreme Court's decision in Utah v. Strieff, 579 US __ (2016). Again, the Court reversed a State court's decision to suppress unlawfully obtained evidence in a criminal trial. The Court identified three exceptions to the exclusionary rule:
First, the independent source doctrine allows trial courts to admit evidence obtained in an unlawful search if officers independently acquired it from a separate, independent source. Second, the inevitable discovery doctrine allows for the admission of evidence that would have been discovered even without the unconstitutional source. Third, and at issue here, is the attenuation doctrine: Evidence is admissible when the connection between unconstitutional police conduct and the evidence is remote or has been interrupted by some intervening circumstance, so that "the interest protected by the constitutional guarantee that has been violated would not be served by suppression of the evidence obtained." (citations omitted)
Strieff was illegally stopped and asked for ID after leaving a house under surveillance by narcotics police. After discovering an outstanding arrest warrant for a traffic violation, the police lawfully arrested and searched Strieff and found him in possession of methamphetamine. The Court found that the exclusionary rule did not apply because of the attenuation doctrine:
The three factors articulated in Brown v. Illinois, 422 U.S. 590 (1975), guide our analysis. First, we look to the "temporal proximity" between the unconstitutional conduct and the discovery of evidence to determine how closely the discovery of evidence followed the unconstitutional search. Second, we consider "the presence of intervening circumstances." Third, and "particularly" significant, we examine "the purpose and flagrancy of the official misconduct." (citations omitted)
[W]e hold that the evidence discovered on Strieff's person was admissible because the unlawful stop was sufficiently attenuated by the pre-existing arrest warrant. Although the illegal stop was close in time to Strieff's arrest, that consideration is outweighed by two factors supporting the State. The outstanding arrest warrant for Strieff's arrest is a critical intervening circumstance that is wholly independent of the illegal stop. The discovery of that warrant broke the causal chain between the unconstitutional stop and the discovery of evidence by compelling Officer Fackrell to arrest Strieff. And, it is especially significant that there is no evidence that Officer Fackrell's illegal stop reflected flagrantly unlawful police misconduct.
- Can the past arrests serve as a basis for reasonable suspicion to stop a person?
- As a basis for inclusion of the person on an informal watch list?
As mentioned in the introduction, this question is not really about "fruit of the poisonous tree." It is an anterior question about whether or not a police stop was unlawful. The court asks: would the facts available to the officer at the moment of the search warrant a man of reasonable caution in the
belief that the action taken was appropriate? Terry v. Ohio, 392 U.S. 1 (1968). Officers are permitted to consider criminal history, as the Fourth Circuit held in United States v. Sprinkle, 106 F.3d 613 (1997):
A prior criminal record "is not, alone, sufficient to create reasonable suspicion." Nevertheless, an officer can couple knowledge of prior criminal involvement with more concrete factors in reaching a reasonable suspicion of current criminal activity. (citations omitted)
- As probable cause for a future search warrant or arrest warrant or wiretap?
- As basis for active surveillance without a stop or arrest or search warrant?
- What if law enforcement set up a sting operation targeted at this individual?
As with reasonable suspicion, evidence of the suspect's reputation and criminal history can be taken into account in establishing probable cause for an arrest or search warrant. However, a history of past arrests would probably not suffice in itself to "warrant a man of reasonable caution in the belief that an offense has been or is being committed": Brinegar v. United States, 338 U.S. 160 (1949).
- Could the suppressed evidence be used to counter an entrapment defense (which requires a showing that the defendant didn't have a propensity to commit the crime)?
- Could the suppressed evidence be used to impeach testimony in a criminal case that the defendant had never used drugs before?
If the evidence is suppressed then by definition it cannot be used adversely to the defendant. However, if the evidence is shown to have been obtained illegally, it will only be excluded where the deterrence benefits of exclusion outweigh its substantial social costs: Pennsylvania Bd. of Probation and Parole v. Scott. The evidence is more likely to be admitted if it falls into one of the three exceptions set out in Utah v. Strieff: independent source, inevitable discovery or attenuation. In considering attenuation, the court will pay particular attention to 'the purpose and flagrancy of the official misconduct.'
Clearly, it is impossible to say, in general, whether unlawfully obtained evidence could be admitted to counter an entrapment defence or attack the defendant's character and credibility. Not only will the application of the exclusionary rule depend on a wide range of considerations, other rules of evidence may need to be applied (such as the rules against character evidence and extrinsic evidence on a collateral matter). Nevertheless, the cases cited above should give some insight into how the court will approach the fruit of the poisonous tree doctrine.