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!)
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!)