Next time run the shorts with the belt installed through the x-ray and show off your clean boxers. You are wearing clean boxers right?
Got a story for that, too. Years ago in some foreign country I had a pair of shorts with a sort of built-in belt. In other words, part of the belt was sewn into the shorts. But again, the buckle was plastic, so I didn't think it would be an issue. When they asked me to remove the belt, I had a heck of a time explaining that the thing was part of the pants, not to mention had a plastic buckle.