Originally, every Muslim is supposed to be honest in all what he finds. He should try his best to make whatever he finds in a public place reaches its owners.
Sheikh Muhammad Nur Abdullah, ISNA President and Member of the Fiqh Council of North America, states: The rules for items that one finds in a public place can be outlined as follows:

1. It is not allowed to take any money found in the Sacred Mosque of Makkah.
2. If you find something valuable, you should announce it. For example, if you find it in the school or beside the masjid or in the mall, then you put a sign there and a contact number for the rightful owner to contact you. It is not allowed in any way to take it.
3. If it is something invaluable, you have two options; either to give it in charity or to keep it.