Gavin de Becker wrote about this in his book "The Gift of Fear." He emphasizes the importance of intuition as an early warning sign. In many cases victims of violent crime mention after the fact that something seemed *wrong* about the perpetrator. Sadly, we're taught to devalue intuition, and the results can sometimes be tragic.
I find this very interesting (which is why I'm responding to this well-aged post.) One of the things I've tried to teach my kids, in order to help them stay safe, is that if someone--anyone--gives them the "creeps," to get the H-E-double hockey sticks away from that person.
I think that most people are capable of tuning in to others and can tell when someone is off kilter, but so many parents teach their kids non-intuitive ways of differentiating between safe and unsafe people and situations that it blunts their instincts for self-preservation. An example: "Give Uncle Herbert a hug, Tommy! He's your uncle--it would be rude not to!" which many parents will tell a child even if Tommy has expressed that this relative makes his skin crawl.
I think people who sense that someone who appears innocuous is a potential danger do so by observing nonverbal cues that are beneath the level of conscious thought--not that there's anything magical about it. Cues might include posture, mannerisms, gestures, muscle tension, even odor. Hmmm...better stop here, my BA in interpersonal communication theory is kicking in! Don't want to bore you guys TOO much.
PS--'Flea, congratulations on a lovely handling of a yucky situation!
Karate Moms have Brass Ovaries!