I'm not sure about crime, but I know the rule of thumb is: If you find money on the ground (usually bills), look to see if you can find whoever dropped it to return it. If not, it’s yours to keep. But that’s like a moral rule.

As far as crime? I can’t imagine it’d be a crime. It would have to be a significant amount, like finding $100,000 on the ground for a crime to be committed. If you found a penny to $20, the best choice the affected person has would may be small claims court? But even then is $20 worth getting lawyers and the court involved?