TSA Explosive Fail

Does anyone else's dog test positive for drugs and explosives every time going through TSA at the airport? My chihuahua used to be a street dog in Tijuana and I get stopped every time I fly with him. After I carry him through the metal detector, they swab my hands, after having just touched the dog, and it always comes up positive and causes me to get a full body pat down and all my bags searched. I asked TSA why and the answers I got range from "He (my dog) must have brushed up against some drugs" to "We detected traces of explosives". I've never done drugs or worked with explosives in my life. Is this unique to my dog or could it be his flea meds (Frontline Plus) that triggers it?