I would return it to the person after asking them if they lost it, if they can tell me how much it is they lost. In fact once I found 2 100 dollar bills at a gas station and I ask everyone their, but no one had claimed they lost any money. So this time I got to keep it.
It's very important to not keep anything that can be returned because you don't know how much it means to the one who lost it. Who knows it could of been part of there rent or for food or something else major. Honesty changes peoples lives!