Third Brake Light Question

[lainmike0710]

Ok so I had noticed this before and hadn't remembered either way what the correct situation was but a 3 series coupe pulls up beside me on the highway beeping to let me know my third brake light has been on for the several miles he's been behind me. I'm assuming it gets brighter when the brakes are being used but is it supposed to stay illuminated at all times and gets brighter when braking or should it be off completely until brakes are hit? Let me know please.