Not in a locker room, but when I was younger and much smaller, I had a friend who loved showing off his cock, he would pull it out all the time, or show the imprint in his pants, or swing it like a pendulum in his boxer shorts, one time he even got it hard.
I had an experience with a HS friend that I also went to college with, so I watched him go from a scrawny kid to College Senior (twink) with a thick 7 3/4" cock. During the interim, virtually every time I went over to his place to give him a lift, he was conveniently in his briefs or quite often, would find an excuse to change his pants.
I sucked it once, while he was driving my car, and had one J/O session in which he measured it, so his size if verified. I was th first person he had any kind of sex with...
He is now married and to this day I wonder why the teasing. I also think if not for having "quite a pole", my memory would have faded. Other than that cock, he really is ordinary, now early 40s ex-twink.