The war on terror has led to much more stringent screening of airline passengers. A small percentage of passengers are selected for even closer inspection, that includes a test for residue of explosives on themselves or their luggage. Such residue could indicate that they were recently working with guns or explosives, and possibly be terrorists. But few (apparently, none), terrorists have been caught with this residue test. What usually happens is that the screeners detect someone who has a legitimate reason to have explosives residue on themselves or their luggage. If the person in question can convincingly identify themselves as a policeman, hunter or target shooter, there is no problem. But sometimes people dont know how they picked up all that residue, and that makes the screeners nervous. For example, some airline flight attendants work flights carrying troops on their way to or from a combat zone. The troops, especially the ones coming from Iraq or Afghanistan, are covered in this, invisible to the naked eye, explosives residue. They have been firing weapons, facing roadside bombs and all manner of explosions, and explosives. The flight attendants, and their carry on gear, get covered with this stuff. If any of those flight attendants next goes through airport security, and gets a residue test, they often have a problem. If they were not told about the explosives residue situation with a plane full of soldiers, then they will have no reasonable explanation for the presence of all that stuff. This usually sends up all sorts of red flags, leads to extended interrogation, and being late for work. Some of the screeners are getting more savvy about this sort of thing, but that kind of experience comes with time. Most screeners are new, because of the federal take over of passenger screening.