Same thing happened to me about a year ago at LAX. I had been on a camping holiday for some time, and I checked in my sleeping bag in its pouch separately. So they swabbed it, and then asked "have you been camping recently?" "yes...?" "because some of the the chemicals used on lawns etc probably set off the machine". They then proceeded to take down all sorts of info on me, and when I inquired as to what would happen to that info post-flight, was given the same answer as studentff....