You're better off forgetting about this Trek, particularly if you have ethics. If the bike is anywhere within the Los Angeles metro area, it means the owner is going to be a pain in the neck to find. And even if you find them, they might accuse you of stealing the seat-post or seat.
Forget about calling the police. LAPD has enough trouble as it is.
The bike is going to be stolen soon anyway - spray-painted and transmogrified into someone's DUI transportation - then casually forgotten about after the "owner" goes to rehab. I'm black-pilled, I know, but I watched it happen many times in Philadelphia when my son was at school in that city.