Maybe before you spend it you should try to find out if the loss has been reported. It could be someone who now has very serious problems as a result of the loss. Maybe it's a family on holiday, and the holiday is now ruined because they lost the cash. They may even be stranded without it.
Losses like this happen most frequently when people are away from home, and they will probably have reported it. Just imagine how grateful they will be, and how their faith in human nature will be restored, if they get the money back.
If despite enquiring it proves impossible to trace the owner, then you could keep the money with a clear conscience. But if you don't make that attempt, you will always have a nagging doubt about it, won't you?