Can someone tell me what it means when a car passes you on the Autobahn and then turns on what seems to be a third brake light? It's a brighter brake light, either below or in the middle of the other brake lights.

Almost every car that passed me did this. I was driving a Volvo, is that why? I was going plenty fast (130-145 kmh) so they weren't trying to tell me to speed up. Also it was on an empty three lane highway, and I was always in the right most lane. I actually pulled over and checked to see if my tail light was out. It wasn't. What were these drivers trying to tell me???