"Mr. Mukerjee appears to have been flagged by the Behaviour Detection Officer (BDO) while in line for what appeared to be unusual behaviours. [...] it is impossible to assess what caught the Officer’s attention, but the TSA source indicates Mr. Mukerjee was already on someone’s radar before he chose to opt-out." What 'unusual behaviors' led to him getting flagged? While maybe Mr. Mukerjee had suspicious behavior I'm…
I sincerely doubt there's a way to differentiate between someone who is ill and someone who is pale and has the jitters because they're about to do something bad. I moreover call foul on the notion that there are enough of the latter set to build an effective training program to create effective BDOs.
What's actually happening here is probably that BDOs are being used to justify retroactively special treatment given to people for invented reasons. It's sort of like a false alert from a drug dog - it's a claim that you can't really cross-examine, since the supposed microexpressions someone exhibits are too fast or too small to be picked up by a camera.