It isn’t jerking off, but once I was using a stall in the men’s room in my department of the university. There are 5 stalls, let’s say going in order 1 to 5, with 1 being nearest to the entrance and 5 being furthest from the entrance. I’m in stall 3, the dead middle.
So while using the stall, I hear someone come in, didn’t think anything of it, he takes stall 5. Let’s call him Guy A. Few moments later another guy comes in, also didn’t think anything of it, he takes stall 1. Let’s call him Guy B. Now, I hate my presence being known when I use a stall, so I’m quiet as a titmouse. Couple of minutes later I hear Guy B whisper “is it safe?”, then Guy A whispers back “yeah”.
Guy B unlocks his stall and walks on down to Guy A’s stall. Few seconds later, I hear a zipper audibly coming down and some wet noises. I look under the stall and I’m not close enough to see exactly what is going on, but I can see legs and shadows. Guy B is on his knees and Guy A is either sitting on the toilet or standing, getting a bj from Guy B. Since each stall has a light directly above it, I can see a full shadowplay on the floor and Guy A looks pretty endowed judging from the shadow cast by his dick. Going by the shadow, I’m pretty sure my dick is both longer and thicker than his, but still his has an impressive size to it.
A few minutes later, some light moans from them, and a frantically whispered “fuck I’m gonna cum”, and they eventually left separately. They never once knew I was a secret audience and admittedly hard as a rock haha.