PB&J? 
 I recently was in the Denver, Colorado airport.  Their equipment indicated a trace of explosive on my hands. They pulled me to the side and looked through my backpack. Again they took a sample from myself and my pack. This time they came up with two explosives. Into the search room I go. A third swipe on my stuff came back the same. 
Intensive pat down and then they removed by backpack and contents from the room for another search. Finally they cleared both myself and my backpack. 
 Everything I carried was identical two days previous when I landed at Denver, except for a lottery scratch-off I had picked up at gas station close to the airport along with a cold sandwich.  Was it the scratch-off or my sandwich? I kept one and ate the other.