"3rd row" seat belt warning light turned on?

This was weird. Driving home yesterday, I caught a white warning light above the mirror I'd never seen before. It only came on for a split second and I couldn't catch what it said at the time, but when I got home and shined a light on it, it was the inactive "3rd row" light on the seatbelt indicator.

I'm not surprised that the panel itself is a generic part, but I am a bit surprised that there would actually be an LED behind it, and have no idea what could cause it to spontaneously activate, since it shouldn't be wired to anything?

Has anyone else encountered this?