It has been said that accepting something not rightfully yours is theft. The problem with this simple is determining when something is rightfully yours.
If someone loses something for 10 years and you find it, is it rightfully yours? What if they die?
Ownership is a trickier subject than is usually realized. The best solution is to allow a free market in the market of property rights. Allow people to organically decide what is legitimate for them, and it will filter out and become a standard in law.