I think that it is actually an open circuit created by the wearing away of the sensor head that triggers it.
My Sister's car was saying that the front pads needed changing after 16k miles. But on inspection the cable to the pad sensor had fractured. Therefore open circuit at the time that the warning was triggered.
Gone: Fiesta - Focus - E46 320dSE - E91 330dMSport - E82 135iMSport - R55 JCW Clubman
Now: M135i with lots of toys