I've just had one of the typical sightings over here: I took the hot sunny day of and went with my wife and our son to the local baths this afternoon. We parked our bicycles between hundreds of other bikes when my eye caught a pretty dirty vintage lightweight nearby. Oh my god..., a mid/late? 1960s Cinelli, old style lugs, flat fork crown, silver with old style decals, early Campagnolo old BCD cranks, high flange record hubs, bar end shifters, 1st gen Weinmann Vainquer 999 centerpulls, Brooks B17,.... too much old grease everywhere, probably not cleaned since 30 years but complete, no rust, no bends, every part correct except a cheap 80s Huret rear derailleur and red plastic (chopper?) brake levers... Obviously not the bike of a collector but one of the many neglected "old bikes" used for the trip to the bath or cinema when you don't want to risk your good bike getting stolen. I seriously considered waiting for the owner but my son got the oh-no-dad-found-an-old-bike-again-look so I attached a note to the Cinelli ("...museum piece....collector....please call..06224..") and went swimming. When we left the baths the Cinelli was gone (of course) with my note lying in the dust (of course).
BTW: Just out of couriosity. What would you do in such a case? Waiting, leaving a note, with/without specific offer? Nothing, just feeling good that such bikes are still in use....?