oh man now I have this problem too! @#$%$#@!!!!
i slammed on the brakes once and it went away for 10 mins and then came back. asked BMW about it and they wouldn't even look at it, said thats just how it is, and they would "hate to charge me the labor hours during the service only to not find anything". so i decided going with new brake upgrades is the solution when spring/summer comes around. bbk are ridiculously expensive so i think I'm going to go with the following setup.
1) R1 Concepts Premium Slotted Rotors
2) stop tech or cool carbon pads (haven't decided yet, any suggestions? i don't track the car only daily driver and spirited driving)
3) new sensors