Where I live, if you find someone else's property and you report it to the Police Department. An officer will secure the property, try to identify the owner and return the property to them. In practice, they'll ask you to drop it off. the Police Department will make efforts
to identify the legal owner of the property and contact them so they can pick it up.
If the owner can not be located, it will be turned over to the Division of Unclaimed Property of the state. Then all other unclaimed property is normally disposed of by public auction.
When I was 14, in California. I bought a 10 speed bike from some guy at the local park for $5. My aunt called the police saying it was probably stolen, police came to pick it up. After some time, I think it was 6 months, no one claimed it and I was able to get it back.
I'd say... "Finders, Keepers" as in international maritime law. Although I may put an ad in paper and craiglist in lost and found tring to locate ligit owner, if they can prove it somehow.