On a CGT...normal (not sure about other models). If you let the car sit, the fluids in the car settle and not enough goes to the brake servo. Pump the brakes a couple of times, drive the car normally, and the warning will reset and go away.

 

Well, thats what i usually did. But after a track session a couple of days ago the sign stays on permanently!

 

Ahhhh...better take it in then to be safe. Good luck! Might want to edit your last statement in case spying eyes are reading...