A little anecdote. I actually had this exact situation happen to me when I was a kid.
I was walking in a crowded street just outside a shopping center and there where a bunch of $50 bills being blowed by the wind. No one was paying any attention to them so I picked one of them up. You can imagine my surprise when I realised that they were real.
All in all it ended up to be roughly $3000 and my mum took the money to the police station. No one claimed it and a few months later we got to keep it.
I was walking in a crowded street just outside a shopping center and there where a bunch of $50 bills being blowed by the wind. No one was paying any attention to them so I picked one of them up. You can imagine my surprise when I realised that they were real.
All in all it ended up to be roughly $3000 and my mum took the money to the police station. No one claimed it and a few months later we got to keep it.