I have an object in my possession that I stole almost 20 years ago. Finding the rightful owner and returning it is impossible. What should I do?

I once lived in a large, very old apartment building, with a bike room in the basement, where residents were supposed to keep their bicycles. The room was virtually unused, as residents tended to keep theirs in their apartments. There were many dusty old unused bikes in there. I cut the lock off one, got new tires for it (the old ones were flat and brittle) and used it frequently while I lived there. I rationalized that a) it was probably abandoned (although I didn't know that, really) and b) the owner was always free to call security, have my lock cut off, and reclaim his bike.

When I moved away, a couple years later, I kept the bike. Clearly I shouldn’t have done so, and I would never do such a thing today. Should I just donate the bike to charity and move on? This is really bothering me.