Finding money and not giving it back to the owner or to the police is considered as theft in various countries. Today there was a report about a Romanian man who bought a cabinet and inside of it he found a hidden box with approximately €59.000 if I remember correctly.
The report was on a German channel and a lawyer explained that in Germany, if you buy an item from someone and find money inside, you have to give it back. Otherwise you could get a hefty fine and even go to prison. You are also entitled to a 5-10% reward depending of the amount found.
If you give the money to the authorities and nobody comes forward to claim the money after X amount of days - the money will be yours.