I noticed a stolen rear wheel from a bike parked over on the SU campus the other day. It was near the art building, and was the bike was locked up onto a rack in plain view. It appears that the owner had only threaded a thin cable lock through the frame, and maybe the front wheel, but had opted out of any protection for the rear. We all know the story, if you don't then listen up - next to the frame the rear wheel is the most expensive part of your bike generally. If you don't want it stolen then use protection man! A U-lock through the rear spokes, collecting the rear frame and the bicycle rack is great. What I also do is use a medium thickness cable lock as a secondary deterrant and lock the bike to the rack with it as well, and include the front wheel for the sake of it (only takes 5 extra seconds).
I actually hope that it was not a stolen wheel and can just imagine that someone had a flat or some other malfunction, and just chose to carry the rear wheel home or to the shop instead of transporting the whole bike. Then once repaired, came back to join up the wheel with the waiting bike right where they left it tied up to the rack on campus. But if it was stolen. better their's than mine.