An ultimately bootless jest, of course. Nothing can possibly alert you to the presence of a ninja except swift and silent death by shuriken.
With the kind of attention to detail (and stress on Entry Control) that they do in BMT, I’d be surprised if a Ninja did manage to sneak in. And once he did sneak in, he’d probably be pelted to death by rock-hard rolls of socks thrown by overly tense trainees. 
Without the Sirens, how will you know when it’s about to change from Foggy Silent Hill to Dark Silent Hill?
Isn’t that radio static? Oh wait, that’s when monsters are near, right?