Bomb detectors are designed to pick up traces of nitrates, the theory being that if you are making a DIY bomb, you're going to go the ammonia-nitrate route.

Nitrates exist in fertilizer - and thats what folks make bombs out of.

Weed is often fertilized with nitrates, and trace amounts of these elements are present in your sweat, in your pee, and so on. When you go through a bomb detector after a heavy weed weekend or so, you're going to be flagged for closer inspection.

This was explained to me by a security agent at an airport I was travelling through after a heavy week in Amsterdam.