This happened at my very first job, the company I worked for was situated in large building that housed various small businesses.
I was in one of the bathrooms that wasn’t specific to any one company (it was closer than our own one). It was just me then some other, older guy from another employer walked in and walked to the urinals, neither of us paying attention to each other.
When I finished I went over to the sinks to wash my hands and as I was a movement reflected in the mirror caught my eye, the other dude had finished and wash shaking his cock (as men do) and the only reason I noticed was because this thing was *fucking massive* and the motion made it easily visible. I genuinely don’t believe he was intending me to notice, there’s no way he could know I was standing at just the right place to see the refection in the mirror. He put it away and I was done washing so I just went back to work. I couldn’t forget that huge cock I’d accidentally spotted though.