The simple issue with the first scenario is really that the police don’t need special permission from defendants or suspects to stake out a bank, I assume they would be setup inside and would need permission of the bank manager–who would almost certainly comply (once appropriate safety precautions had been taken) if told his bank was going to be imminently robbed.
The actual conversation the police captured via wiretap would not be admissible in court, but the evidence they collect at the bank would be. The excellent HBO series The Wire actually commonly mentions the practice (which is used in the real world) of actually calling wiretap information “anonymous tips” or “anonymous confidential informants.”
There’s a bit of a two-sided coin to that strategy. Firstly, nothing given to police anonymously can be impeached in court, which also means it cannot be used as evidence towards a defendant’s guilt. In many cases a wiretap might generate some interesting information that can be used to affect an arrest, but the conversation recorded isn’t that material to securing a conviction. In such a situation, it is better to preserve the secrecy of the wiretap’s existence than to expose it needlessly.
On the flipside, this information by itself isn’t always going to be enough to do much with it. The bank robbery scenario is a good one because the police just have to go set up in a bank, something they can do without any special permission.
Now let’s say the police are operating an illegal wiretap and get information than John Doe has a hidden safe in his house full of cocaine. This is a little trickier because they have no easy access to that safe without a search warrant, and they have to go to a magistrate or judge to get a search warrant. They don’t want to admit they are operating an illegal wiretap, so they might attempt to use an anonymous tip or confidential information. That’s problematic on a few levels.
For one, actually lying to a judge in your search warrant application, if ever proven, is going to result in significant criminal and professional trouble for the police officer.
For two, there’s a couple Supreme Court precedents, that in totality, general make it difficult or impossible to get a search warrant solely based on an anonymous tip or the word of a confidential informant not willing to be identified or made available to the court.
The first one is the Aguilar-Spinelli test, named after a couple of cases in the 1960s. This basically lays out that there are circumstances where a judge can properly issue a warrant based on an anonymous tip, but the police have to explain to the judge the reason they believe the tip is reliable along with other supporting information. In the case of a confidential informant, this might mean a detective has to go talk to the judge about the CI’s history with the department, how reliable they have been in the past, and other supporting information. This makes it much harder to use a CI to cover up an illegal wiretap because it requires fairly deliberate lying to the court (this actually happens in The Wire too, the infamous Fuzzy Dunlop CI that is used by a couple of the officers running a scam to get reimbursed for some expensive camera equipment they had bought on credit and subsequently lost.)
In the traditional Aguilar-Spinelli test, at a post-arraignment hearing, the police have a specific requirement to show facts that their information is reliable, and secondly to explain the circumstances surrounding the person’s access to the information and how the person making the information available to police came to rely upon it. This basically means that while you can use a person who is “anonymous to the courts” it is not really easy to use a non-existent person or a person anonymous to the police.
The Gates test is the result of a 1980s case in which the limits on confidential informants / anonymous tipsters were loosened slightly. In some ways the Aguilar-Spinelli test remains the basis as it is somewhat encapsulated in the Gates test, and some states still specifically require the elements in Aguilar-Spinelli, but the Gates test allows for an anonymous tip to be used as the basis for a warrant that would otherwise fail the Aguilar-Spinelli test, if, based on the totality of the circumstances.
The Gates case arose from a situation where someone wrote a highly detailed letter detailing the drug sales operations of a couple, including evidence that police could use to easily catch them in the act. The police then did an investigation, where a detective followed and observed the couple for a period of time, documenting things they did, and building up further evidence. A warrant was then issued, and the couple was caught dead to rights.
Lower court rulings held that the nature of the anonymous letter failed the Aguilar-Spinelli test because unlike an anonymous tipster or a CI, the police couldn’t really explain a basis for believing the letter was reliable when they began their investigation. This wasn’t a person known to them or a person the police could vet, but just literally a letter someone dropped in the mail. Over the course of appeals, it got to the Supreme Court. The Supreme Court ruled that the letter by itself was indeed not sufficient, but that the police had done a supplemental investigation that demonstrated the letter’s reliability and veracity by corroborating much that was in the letter through stakeouts and observations–which themselves didn’t require special privilege for police to perform. By the time the warrant was issued, the Supreme Court ruled that the totality of the evidence was that it was reasonable to rely on the letter.
So TLDR–if you operate an illegal wiretap and then report its results as an anonymous letter, under the Gates test it would be quite easy to use that as a basis for a search warrant–but you would need to have done some more case building, which often times if you were operating an illegal wiretap you’d have some knowledge maybe of what you could do in order to build supplemental evidence.
However the hypothetical I posed of a guy with a stash of cocaine in a secret case is a harder nut to crack, he would have to be observed doing something that the police could use as supplemental evidence supporting the claims in the anonymous tip (aka actually what was heard on an illegal wiretap.) Depending on the case this may be trivial or impossible, and in cases where it would be impossible that’s unfortunate for police because if they had a legal wiretap they could just submit it directly as justification to go in and search the safe.