In terms of showering and seeing things, a Headmaster, who as you describe made a deliberate note of when a boy was next going to do physical education, and who then deliberately made sure they just happened to be around the showers at the end of that lesson – again, he would be likely to attract suspicion. Doing so, very occasionally, might not do so – if you knew it wasn’t his normal practice, you might think that perhaps on this occasion, he was worried he’d caned a boy too hard. If he did it at all often then opinions would probably be less likely to allow for that possibility.
But there are other scenarios that are less suspicious. At the time when I caned boys on the bare buttocks – and let me stress I did it less than half a dozen times in extreme circumstances, I was Deputy Boarding House Master and then Boarding House Master of my school.