I set off the full body scanner when I went through security at the airport. After double-checking that my pockets were empty, I went back through hoping it was a fluke, but I set off the alarm again. The TSA officers were as confused as I was because they'd seen me pull out my pockets and knew that there was nothing there. So, they gave me an obligatory full-body pat down, yielding nothing, and let me through.
In hindsight, I wonder whether this had to do with my clothes being wet. It's possible that wet clothing refracts differently, making the scanner less confident in its findings. So, to err on the side of caution, it raised an alert. Anyway, next time it's rainy, I'll be sure to take off my wet clothes before going through security just to be safe.