"Agents Scully and Mulder, please come to the restroom..."

A few minutes ago, I had to go to the restroom. As I approached, I heard a familiar whirring sound through the wooden restroom door. It was the new motion-sensing paper towel dispenser, dutifully dispensing a length of towel.

However, when I pushed the door open, there was nobody in the restroom. Nobody had just come out, and there were no feet under the stalls. There were no insects flying around. It was empty except for me.

So, what in the heck triggered the motion sensor?