I agree you don't need metaphors, but not because it's not like anything, rather because it's too simple to need a metaphor: If you give something, anything, to a third party to handle or keep, the government can't demand to go and look at it without a warrant.

The government can't demand to go look in your safety deposit box, it can't get your library check out records, it can't see how many airline miles you have, it can't read the mail in your PO box etc etc without a warrant. Of course, in some of those cases, the third party will cooperate without a warrant, but they're not required to.