That's another phrase I don't understand. How often do you hear it? You hear it quite often don't you.
Usually after the police have captured a serial killer or gun totting, knife wielding lunatic journalists will descend on his neighbours and interview them to find out what kind of guy he was - as if it makes any difference. Often the neighbours might say "Well he was always really quiet, kept himself to himself mostly, never heard a peep from him, seemed like such a nice guy". There is then usually someone within earshot who will say "Aye, but it's the quiet ones you have to watch". This is a stupid thing to say and needs to be completely rethought. For one thing quiet people may get offended with such a statement - and you don't want that.
But really in all honesty it needs rethought because its not always true. For example I'm a quiet person by nature and despite my weird tendency in this blog to focus on the negative I am a fairly positive thinking person. I focus on the negative here because it amuses me, not because I believe the crap that I say. I don't need to be watched and really by doing so you'd be wasting your valuable time.
Now if on Sunday I bought a paper, sat in the corner table of my local cafe and enjoyed my lunch without saying a word to anyone and some other person grabbed a knife and shouted at the top of his lungs "I'll kill the next fucker who gets served before me" in that situation I really feel I wouldn't be the right person to watch. Of course that's only my opinion. Any person who watched too many documentaries about quiet serial killers may think differently.
But really despite my weird tendencies I am harmless. Infinitely more harmless than anyone who grabs a knife due to being the recipient of lousy minimum wage service.
As statements go its a dangerous presumption to believe, it might get someone killed. Hitler wasn't exactly quiet in public, he was loud and forceful and by all accounts a bit of a nutter. Personally though I wouldn't even presume that all the noisy ones need to be watched. People deal with things differently, after letting loose the guy above might well feel better for getting his displeasure off his chest. Not every noisy person is going to turn into a Hitler, and not every quiet one needs to be watched. In fact it might only piss some of them off.