Prescription drugs?

I’m at an airport right now. I just watched as security people swabbed a traveller’s carry on luggage, “personal item”, the plastic bag that contained her liquids, etc. and tested everything right there. I’m pretty sure the swab test could easily pick up high levels of THC.

When I've seen swabs done it was for explosives. Explosives are also the reason for 'small liquids' rule.