CBS doesn't work as well as it should. The reading of miles left is a guesstimate until the sensor is triggered, because there's no actual mechanism to measure the pad thickness. My iDrive said I had 40,000 on my rears, until the sensor went off, and then it went to 1,000. My experience:
http://f30.bimmerpost.com/forums/sho....php?t=1471617
The only way to know the condition of the pads is to inspect them, all eight of them. If you had a sensor trip the sensor has to be replaced, whether the pads are replaced or not, so unless you're capable of doing your own work a trip to either the dealer or an indy is inescapable.