I'm asking this hypothetical situation mostly as I was considering writing a story with the premise and want the story to be legally accurate, but also because I'm frankly curious how the law would handle such a situation.
Let's say someone (call him Bob) got sick of police being unable to act against individuals everyone 'knows' to be committing a crime due to lack of sufficient evidence to get a warrant. One day he decides to take matters into his own hands and he calls into 911 with a made up claim that he witnessed a crime take place and a gunman drag someone into a known crack house.
As I understand it when the police arrive they will have sufficient justification to enter the crack house without a warrant to intervene with a crime believed to be in progress. They won't find evidence of the original, made up, crime, but once they have entered the house they will likely see evidence of other crimes. Since they were there in good faith the evidence of other crimes within 'plain sight' in the house would be fair game and could be used to make arrests and/or get a warrant to search the rest of the house for more evidence. Despite the fraudulent nature of the original claim arrests could be made since Bob was not an agent of the police. So far this is covering legal concepts I think I understand.
Now lets say that the police were never able to trace the original fraudulent call to Bob, and Bob gets it in his head to act as a the world's least interesting vigilante. He starts regularly making fraudulent calls claiming to witness a crime in progress to the police to give them justification to enter locations that are commonly believed to have illegal activity within them in a hopes of helping the police to collect enough evidence to make arrests.
While Bob changes the number he is calling from and as much of his MO as he can each time the police would noticed a pattern of fraudulent claims and have reached the point that they can usually guess when a call is a fraudulent one made by Bob, but they can never be certain it's him or that a crime isn't actually happen. Assume the police have actively stated they do not support or condone Bob's actions, have made a public plea for him to stop, and are trying (but failing) to find and arrest Bob as well.
I assume the police would continue to respond to the claims in case the call is real and a crime is actively in progress. Is there a point where they would no longer possess sufficient cause to enter a premise to investigate a believed crime in progress due to their suspicion the crime may not be legitimate? If they do have justification to continue to enter the premises then at what point, if ever, would evidence witnessed as a result of responding to the potentially fraudulent calls not be admissible?