That raises an interesting “what would you do” question. How would a finder identify it as a police weapon, until it was announced to the public that it was missing? Would you run a “found” ad in the paper / on the ‘net, with a request for the owner to form a line on the right and don’t push? If you had it for a day or two before you became aware of its origin, then turned it over to police, would you be in trouble? If you find a weapon and turn it in to the police, you can pretty much bet that you’ll never get it back!
It also raises an interesting question about the consequences to the police officer, should the firearm be used to perpetrate a crime. The officer was negligent, already, all that’s missing is damages.
Wait for a gun buyback and turn it in then. Only way to ensure it is destroyed and doesn't find it's way back onto the street.
No good whatsoever can come from just calling the police station and arranging to bring it in, or even for them to come out and pick it up. I can well imagine that in many jurisdictions they could and would gin up any number of charges on you; “possession of stolen property” is pretty much a given.
If I found it, I wouldn’t keep it. Too much trouble if you got caught with it. I’d sell it to a gangbanger instead :)