There's no reason why the police have to search your backpack (without a warrant) after they've already released you from the police station. None. Zero.
The Supreme Court has said it's fine for the police to search your backpack after you're arrested as part of an "inventory search" -- that way, they'll know what's in the thing (e.g., nothing dangerous) and you can't say that the police stole your stuff. That's fine. They can't give the thing back to you (since you're in jail) sight unseen, so an inventory search is perhaps justified.
But if they've arrested you and already released you, they can just give the thing back to you. There's zero need to search it -- much less any need to do so without probable cause and without a warrant.
But the Ninth Circuit says it's just fine to search the thing anyway.
I'd have joined Judge Hawkins' dissent.
