Hand it in.
Hubby has found quite a few phones on his walks....not so many now, perhaps because they cost so much people are more careful.
I have had chocolates bought by one happy chappy...nothing for hubby
We found a purse in the middle of the road when out shopping, uni card in it so we rang the uni,they got in touch with the young girl and her dad came and picked it up. Bottle of red wine in hand as a reward I told him to share it with his wife over dinner
If I found cash in the road then I would keep it.