like dice said, they just need "probable cause." which basically is the "wink wink, nudge nudge" way of saying they can do whatever they want. if they really want to get through the door, they can break it and then say they heard a scream, or smelled weed, or something like that if anyone questions them afterward.
I had a cop give me a breathalyzer because he said he 'smelled alcohol.' It was 9:00 in the morning and I'd been nowhere near alcohol. He was just fucking with me. I don't trust the fuzz.