So there I am at BWI Airport on the way to Las Vegas. In my Pelican was a Nikon F, a couple of lenses, and 2 rolls of Tri-X in a X-Ray proof bag. As expected, the person looking at the monitor calls for an inspection after seeing the bag. No problem. So they do a swab on both rolls of Tri-X and one comes up indicating explosive residue. These were unopened canisters. They run the "explosive" one through the x-ray machine again and all is fine. While this is happening, I get the new TSA pat-down. Everything turned out OK and the TSA folks were professional, courteous, and let me bore them to death about my '67 F after they asked about the "electronics" in them. I guess I'll start shooting with the roll they didn't run through X-ray again. My only question is what was on the Tri-X that set off the machine?