I've seen this cliche a hundred times and this is one of the worst, MC heard the girls, could have just yelled he was in, they would have realized the old man at the entrance made a mistake and laughed it off.
This lucky pervert moment was EXTREMELY forced, as I said I've seen it a lot, and other times the guy NEVER hears the girl in the dressroom, is distracted, away from the entrance, playing underwater, they had a reason to not notice the girls, some even stay just to enjoy the view, this guy was forced from the author in a pervert position.