Yesterday I saw something on a gasoline transporter that I had not noticed before. It was a LED light that I assumed was a third brake light centered on the back of the trailer.

But it was flashing in a quick sequence. Then the truck stopped for a traffic light and the brake lights, separate from this light, came on like a regular brake light while the other light flashed.

When the truck accelerated from the stop light both the brake lights and the flashing light went off. My thought was this was a form of warning light connected to a engine compression brake or Jake brake. When the truck slows down without its chassis brakes from the compression brake this light would signal that it was slowing down. It flashed so it would be different from the regular chassis brakes being used.

Is this a good guess or is it something else?