Went out to a Mercedes Benz, fault was brake light bulb failure. However the brake lights all worked fine. Traced the fault down to the brake light bulbs being burnt slightly. They still worked fine but the car ECU saw this as a fault. Replace the bulbs, job done. This is happening more & more on all makes of car with bulbs that are still functional but have started to burn out. If you do get this warning come up then it is worth checking your bulbs 1st.
Stu