The Secret Service didn’t catch Tareq and Michaele Salahi, says martial artist Stephen W. Browne, because the couple didn’t appear to be up to anything sneaky:
Presidential security, any security organization charged with protecting life and property, is trained to perceive and deal with threats. A threat, to bodyguards, is most often a person or persons nearby with the intent to do harm. That intent creates in an aggressor, certain subtle patterns of behavior that people with experience and competent use-of-force training learn to recognize.
And the Salahis didn’t exhibit any of those patterns:
The Secret Service fell asleep on this one precisely because [the] Salahis weren’t assassins, spies, or saboteurs. They weren’t on a mission they were on a lark.
The Salahis were completely without malice, and thus failed to alarm the trained “instincts” of the President’s bodyguards.
Apparently this is a job for the likes of Sherlock Holmes, who is attentive enough to note when a dog fails to bark.