The swabs are looking for mercury as well as nitrates and things. A small bit of mercury can do a lot of damage to an airframe - nitrates too of course, but in larger quantities.

If you've been around mercury vapor, you'll set off alarms at the airport. Old church attics are often quite heavy metal.

Also, airports in places with free/open gun culture tend to have their weak spots, I'd imagine, when it comes to false positives ..