"Finders Keepers, Losers Weepers!" Have you ever found something that belonged to another and decided to keep it, using that little ditty to salve your conscious and justify your actions? Actually, under law a finder does have some rights to found property. If it is abandoned or lost he has a greater claim than most. The landowner where it's found could claim a right to it, and may have a greater right than the finder. But again, the true owner always has the greatest right to his own property, which creates an obligation on the finder to return it.
This goes all the way back to the Law of Moses (Deuteronomy 22:1-4). In the text both ideas are noted. If I find something that belongs to another I'm obligated to return it. No hiding it; no exceptions, no excuses. But, if I don't know the owner I must take it with me and keep it until the owner comes looking for it. Again, the principle is that it’s his property, not mine. That means there's an implied duty to attempt to identify the owner. Also by implication, if the owner cannot be identified and never comes looking, the item becomes the finder's.
Can you see a connection to the second greatest commandment (Matthew 22:39)? How can one love his neighbor as himself if he takes his neighbor’s property? Beyond that it is built on the greatest commandment (Matthew 22:37). My neighbor belongs to God and I should never want to hurt God by harming my neighbor. We are lost property because we wandered off and were stolen by Satan and sin, but Jesus came to find us (Luke 19:10). Will we let ourselves be found or keep in hiding?