> If you set off a gas chromatograph for nitroglycerine and then say you have heart tablets then you'll be fine. He obviously set off a specific trigger AND couldn't account for it (although they clearly tried to get him to).

That isn't what that line of interrogation was for. The TSA goons probably don't know anything about chemistry or how their machine works, they were simply trying to rattle him and get him to say something incriminating.

Specifically: when somebody says, several different ways, "why are you setting off my explosive detector?" what you do not want to say is "I don't know! I don't have any explosives!" Because it will elicit a response indicating that nobody was accusing you have of having explosives, why are you being so defensive, etc. etc.