I'll say it again. The sensitivity of the sensors does not turn a light on untill the car is moving. Look only at the resistance of the sensors if your light comes on while sitting. Either that or look to some form of induced electromotive force (I've heard a CB radio will set the speed counter). With a scope you should have no activity on the sensor leads at rest. You get a AC sine wave style pattern with variable amplitude and frequency as the the speed ring teeth pass the blade of the sensor. Distance matters to amplitude but not frequency. The controller is more interested in frequency than amplitude but amplitude variations will set the light.
There is no amplitude or frequency to be judged at rest, unless its a stray signal.
Bosch Master, ASE Master, L1
33 years MB technician