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My dad flew on the 5th of July a couple years ago. While going through security, he commented that they must get a lot of false positives around that time of year. Their response: "We don't test for black powder."

My dad had food (dates) set off the detector. He's now on the no fly list.

It's hard to explain a false positive when you're given no context about what it could be.

As explained in other comments, nitroglycerin is found in things as bizarre as desserts.

Unless someone happens to be incredibly knowledgeable about how to make explosives out of common household supplies, it's hard to expect them to say, eg. "Oh, that's because I happened to eat dessert last night and forgot to wash my hands."

(In fact, if somebody does know that, that may be cause for all the more suspicion!)