Saw an unlocked locker at the gym a few years back, which had a few items in it. A pair of tighty whities were prominently displayed on top and I couldn't help myself. Ended up throwing those on during my workout and I stayed hard the entire time. They smelt great and had definitely been used recently.
Another time I also found a pair hanging on a rail in the handicap stall at a different gym. The guy who left them obviously left them there on purpose, as there was a nice scent and visible stains all over them. I took a huge whiff and jacked off into them. Once i was done I left them for the next guy ;D