It’s a training unit. It trains recruits. That’s pretty visible. A training unit that is spending its time not training recruits would also be pretty visible.
So consider their job description and tell me how much of that they do where you can watch them.
Of course, this is a silly argument, since if they were hiding their real activities, you would know nothing about it. Your claims that they couldn’t be doing anything covert is absurd, since you have no way to see what they do either way.