He was holding the camera in such a way to try to avoid seeming obvious. Like he was trying to conceal it. He appeared nervous, and his body language was shrunken. He was taking pictures of the interior buildings of the airport. Nothing scenic about them, really. Not looking *out* at the airplanes.. but inside the loop, towards dingy concrete buildings. This was not a plane spotter. He appeared to be an international traveler, but his bag didn't have a scuff on it. His clothes were fresh, clean, and without wrinkles. He didn't look road weary or anxious about where he was going or what time it was. In fact, he rode the train nearly all the way around the airport. Nobody pays that much attention to the dirty interior buildings of DFW. It was weird.
More than anything, I went with gut feeling. The guy seemed like the type who really didn't want people to ask what he was doing. So I tapped the TSA to do exactly that- investigate and ask what he was doing. The problem here wasn't necessarily my judgement- it was the TSA's total lack of action regarding a potential intelligence gathering operation.