Losing or misplacing one’s property constitutes a relinquishing of rights of sorts, in the sense of losing control of the property. Such relinquishing is to the point that it is by the finder’s good graces that ownership can be restored to the original owner.
However, a finder will also be judged by others according to the finder’s attitude to finding something that can yet be traced back to the owner and thus returned. Even if not an outright thief legally speaking, the finder who insists on holding on to the found item can still suffer social backlash, not to mention be tracked by the original owner who would be in his right to exercise non-coercive pressures.
Ultimately, peace and order come about from a system where accountability is maximized, where one faces the consequences of an action, even if only in the way the rest of society treats them, let alone how coercive entities (gangs, governments, etc.) bestow sanctions. The actual legal nuances as to what constitutes property is secondary to this even.