I'm talking about American jurisprudence. Police (to include CPS) can pretty much do anything they wish. They are, after all, the ones with the most weapons. But, taking that evidence they've collected either with a fraudulently obtained warrant, or no warrant at all, results in exclusion at trial - always.
To your example about "calling in a anonymous tip", the law is clear in this regard. In every Circuit in the country, probable cause cannot be established with an uncorroborated anonymous tip. Without probable cause, no grounds for a warrant. No warrant. No admissible evidence.
The Supreme Court held just a decade ago similarly, when they disallowed frisking someone SOLELY on the grounds of an anonymous tip. That search was ruled to be inadmissible because the police corroborated nothing in the tip before performing the search.
Pollywog, I’ll try to leave this thread in your hands. It riles me to see conservatives, sensible, govt-wary folks usually, being so naive. I don’t think I can hang on to my usual polite standard in the face of such...