These are not exactly public thoroughfares but there are presumably athletes going in and out of the showers from practice sessions all day long!
So why did this guy think he could get away with the practice?
One conclusion is everybody else he encountered was of the same inclination.
At least for the attack witnessed by McQueary, it was long after anyone would normally have been around: 9:30 PM. The locker room was dark, and McQueary noticed the lights on in the showers, which was unusual.