Back when I was in college, in a relatively small college in Ireland, I went to use the restroom nearest to the canteen one quieter day near summer. I went into a cubicle stall to pee in the toilet, when I was done and turning to leave I nearly slipped on something. I looked down and saw drops or puddles of something, it was slippy not like the consistency of water, they seemed to be in a line, my eyes followed the line from the center of the cubicle floor to the side wall and up the side wall. I was a bit more naive then than I am now so it took me a moment to realise what it was. Someone had clearly leaned on one side wall and rubbed one out, shot his massive load onto the other side wall creating what looked like a target at waist height, and then the remaining ropes of cum painted the lower parts of the wall and floor and then just zipped up and left. I was so incredibly turned on because this was still very much liquid and runny and must have just happened moments ago before I stepped into the restroom.
It clicked with me then that I realised I had seen dried on cum in cubicle stalls before. In the same college there are larger sized accessibility stalls next to the standard stall in the same restroom but the accessible ones have a sink inside the stall and full length mirror on the back of the door. Those mirrors were always getting covered in something and now I guessed what.
From that day on I took it upon myself to have a wank in every toilet cubicle stall in the entire college and I'd always wait for another guy, or better still, a bunch of lads to come in before I'd get going, thinking about them at the urinals with their cocks out while I'm going at it, jerking in the stall just next to them was amazing.
I didn't end up leaving my mark anywhere like the one I originally found, I'd shoot into tissue or into the toilet, although if I missed I'd leave some of those bits as they're behind the toilet not where anyone could slip, but working through every cubicle was fun.
Even now every time I use public toilet I check for cum stains, and use the light of my phone as some colours and textures on walls can hide stains in plain sight.
