I was once pulled aside for secondary screening by the TSA, and I was so bewildered that they asked if I'd taken a cab to the airport—and I had. The agent expla

I was once pulled aside for secondary screening by the TSA, and I was so bewildered that they asked if I'd taken a cab to the airport—and I had. The agent explained that it's a common cause of false positives like mine—"residue" gets left behind on the seats. I didn't feel like pressing them for more details, though.