Main/Search Warrants for Touch ID/c000052

I believe implicit in Kerr's example of Case #1 is that the police know that it's the defendant's phone, which is what distinguishes it from Case #2. There is an exception to the warrant requirement -- the technical name escapes me -- for something that merely reveals what is already known. Now that's going to bug me all day until I can remember what the name of that exception is.