Either call them look them up people are not hard to find! or take it to your local police station and just tell them you found it in the street its not a big deal.
Their not going to ask you anything just tell then you found it. They aren't going to arrest you
I found one and called the person it ended up being an older couple who lived quite some distance from me they made arrangements to come get it and all was good, they were just happy to know were it was and not floating around someplace, just a piece of mind knowing someone else didn't have it any longer.
They send us a few Christmas cards after that just because all was good.
Just return it any way you can or want its no big deal.