At the risk of sounding like an ass, doesn't Mycroft's ability just seem like blatant metagaming? It feels offensive to the character's creators having hidden information revealed just because someone with no prior knowledge looked at them, and it undermines the idea of creating connections when someone can just find out whatever they want at a glance.
In Syler's case, she has to put in the effort of stalking them as inconspicuously as possible (or harassing them for information), but Mycroft just magically "knows" with no consequences, just because he could stare at them for a moment.