According to the agency:
How does the Secret Service "protect" the president? In order to
maintain a safe environment for the president and other protectees,
the Secret Service calls upon other federal, state and local agencies
to assist on a daily basis. The Secret Service Uniformed Division, the
Metropolitan Police Department, and the U.S. Park Police patrol the
streets and parks nearby the White House. The Secret Service regularly
consults with experts from other agencies in utilizing the most
advanced security techniques. The military supports the Secret Service
through the use of Explosive Ordnance Disposal teams and
communications resources. When the president travels, an advance team
of Secret Service agents works with the host city, state and local law
enforcement, as well as public safety officials, to jointly implement
the necessary security measures.
The same source recites this authority for Secret Service agents:
Under Title 18, Section 3056, of the United States Code, agents and
officers of the United States Secret Service can:
- Carry firearms
- Execute warrants issued under the laws of the United States
- Make arrests without warrants for any offense against the United States committed in their presence, or for any felony recognizable
under the laws of the United States if they have reasonable grounds to
believe that the person to be arrested has committed such felony
- Offer and pay rewards for services and information leading to the apprehension of persons involved in the violation of the law that the
Secret Service is authorized to enforce
- Investigate fraud in connection with identification documents, fraudulent commerce, fictitious instruments and foreign securities and
- Perform other functions and duties authorized by law The Secret Service works closely with the United States Attorney's Office in both
protective and investigative matters.
Additional authority is found at Section 3056A. Some of the agency's enforcement philosophy and approach is explored here.
It isn't mentioned in this short blurb, but law enforcement officers, in general, also have authority to seize contraband and various money and equipment used in furtherance of criminal activity under civil and criminal forfeiture laws. See generally 18 U.S. Code §§ 981 and 983 (civil forfeiture) and 21 U.S. Code § 853 (Criminal forfeitures).
Thus, a Secret Service agent really doesn't have particularly much more authority when acting in a protective capacity (Secret Service agents also serve a detectives investigating counterfeiting as a consequence of their historical ties to the Treasury Department).
In particular, any special authority of a Secret Service agent acting in a protective role either:
(1) flows from permission (lots of law abiding people will cooperate with the Secret Service even if they are not required by law to do so in part out of respect for the institution of the Presidency and the President's symbolic role as head of state, and most of the time the President has the express permission of a venue owner and his entourage to be present there), or
(2) flows from laws that are phrased in terms of making it a crime to do certain things in the presence of the protectee or at a particular venue, rather than in terms of granting special authority to the Secret Service in particular, which makes sense as a way to structure these laws given the heavy reliance of the Secret Service on cooperation from other law enforcement agencies to do its work, for example the statute that makes it a crime to threaten certain protected persons (18 USC 871) or
(3) flows from the affirmative defenses to criminal liability for conduct that is privileged for self-defense or the defense of others by a law enforcement officer, or
(4) with regard to searches, but not necessarily seizures, there are national security doctrines that authorize searches for purposes of intelligence or national defense purposes (which protecting a protected person would normally include) as opposed to for the purpose of preventing or investigating a crime. Usually information secured by a search under this kind of authority would not be admissible at trial in a criminal case based upon information discovered in such a search, but the Secret Service may not really care if it can get a valid conviction on the basis of that particular evidence so long as it can protect the protected person at that moment in time, or
(5) in cases of ambiguity or lack of very specific legal precedents, Secret Service agents have "qualified immunity" from civil rights liability (which is all they need if they are not seeking a criminal conviction) for their actions since they are not violating clearly established law. So, they have the capacity to press their advantage in cases where they no that no court has ruled on a factually specific situation similar to their own.
Also, keep in mind that Secret Service agents acting in the course of their federal duties are generally immune from civil or criminal liability under state law and can only be criminally prosecuted under federal law if the Justice Department (which reports ultimately to the President who has a vested interest in having the people that the Secret Service provides protection to being kept safe and has a special relationship with federal law enforcement agents), in its sole and absolute discretion, decides to prosecute the Secret Service agent. The President also has the power to pardon a Secret Service agent for a federal crime. Thus, the only practical means of enforcement of limitations on a Secret Service agent's authority that a President approves of is a civil rights lawsuit under 28 USC 1983 brought in federal court (to which the qualified immunity defense will often applly). Also law enforcement employers routinely indemnify and defend their agents when they are sued for civil rights violations where the employer don't personally think that the agent deserves to be sued even though this is not a binding obligation unless a union contract says so (I don't recall if Secret Service agents are unionized or not).
Permission is buttressed by the criminal provisions of Section 3056(d) which state:
Whoever knowingly and willfully obstructs, resists, or interferes with
a Federal law enforcement agent engaged in the performance of the
protective functions authorized by this section or by section 1752 of
this title shall be fined not more than $1,000 or imprisoned not more
than one year, or both.
This criminal provision was enacted in 1971.
Some states, such as Colorado, also give Secret Service officers by virtue of state law all of the rights and privileges of a state law enforcement officer to enforce state laws.
I suspect, but do not know, that there are regulations issues by the Department of Homeland Security and/or the Treasury Department where the Secret Service was previously housed, that interpret its statutory authority broadly and are entitled to deference in interpretation of ambiguous aspects of federal laws under the Chevron doctrine. My suspicion is based, for example, on the way that the agency handled the GOP National Convention in Cleveland, but I can imagine other paths (e.g. putting pressure on event organizers) that could have led to the same result.
In general, the Secret Service gets to have final say when multiple agencies are involved, on designing security procedures for events and for individuals within their jurisdiction. This has the following source:
The Secret Service’s authority to do this stems from a 2012 change to
US Criminal Code, 18 US Code § 3056 (e) (1), which reads, “When
directed by the President, the United States Secret Service is
authorized to participate, under the direction of the Secretary of
Homeland Security, in the planning, coordination, and implementation
of security operations at special events of national significance, as
determined by the President,” and Presidential Decision Directive 22,
a secret directive issued by President Obama in 2013.
In the linked story, it used its authority to establish a press credentialing and background check process at events where protected persons would appear.
With regard to your specific questions:
Do they get warrants for those searches, and/or for any seizures that
may be associated with them? What could they do against people if they
Secret Service agents have the authority to request and enforce search warrants, which must be issued by a judge on the basis of an affidavit or declaration establishing probable cause of a search.
But, there are a variety of exceptions to the warrant requirement of the 4th Amendment (e.g. "Terry stops" which require mere reasonable suspicions, searches with permission, searches and arrests for crimes committed in the presence of the officer, searches incident to an otherwise lawful arrest (either due to a warrant or based upon probable cause to arrest even if the offense itself does not have incarceration as a potential punishment), searches based upon probable cause when exigent circumstances make it impossible to obtain a timely warrant which often apply in "active shooter" or hostage situations and to most searches of cars).
- Handgun that is generally legal to own
- Hunting rifle designed for accuracy at long distances that is generally legal to own
These kinds of searches and seizures would have to rely on (1) a prohibition of a venue owner in which case trespassing would provide a criminal law violation, (2) a statute particular to acts in the presence of a protected person, or (3) creative interpretation of laws that, for example, prohibit "menancing" with a weapon or involve "attempted assault or attempted murder" on the theory that under the possession of the weapon in the known and anticipated presence of the protected person was a concrete step towards an intended use of harming the protected person or another person.
Note that a Secret Service officer cares about the arrest much more than the conviction given his or her mission, so they care more about having "probable cause" that an attempted crime is being committed, which is grounds for a lawful arrest or lawful, than about being able to prove that a crime was committed beyond a reasonable doubt. The definition of probable cause is beyond the scope of this answer but is a much lower standard than even a preponderance of the evidence that applies in non-criminal cases incluing civil forfeiture cases.
- Assault weapons that are not generally legal to own
- A throwable explosive such as a hand grenade
- Other explosives
The Secret Service has authority to both investigate and enforce criminal laws pertinent to its mission including a law enforcement right to make civil seizures of contraband. All of these things would generally be illegal to possess without special licensing that may itself impose restrictions on its use.
Marijuana, which is federally illegal, in a state like Colorado
Harder drugs like cocaine or heroin
These are federal crimes that any federal law enforcement officer can enforce, except that there are temporary budgetary restrictions that prohibit using federal funds to enforce marijuana law violations that comply with state law. Since these are outside the mission of the Secret Service, it can't actively investigate these violations but can use all of the authority of any law enforcement officer including seizure of contraband when encountered in the course of other activities within their jurisdiction or if in a particular case enforcing the law would further the purposes of their jurisdiction.
Counterfeit US currency, along with a printing press and materials for
This is contraband in violation of criminal laws that can be searched for and seized, and is within the jurisdiction of the Secret Service for investigative purposes.