This is a real coincidence this. I did the Front discs and a full pad change on a 330ci sport a couple of months ago. I got all the pads and the rear sensor from GSF (as I was passing there) and the front discs and the front sensor from BMW (as I found they needed replacing once it was stripped). The owner has come back saying the warning light comes back on and off intermittantly. She can't pinpoint when it does it. theres no pattern.
So yesterday I have it back in and run a few checks. Went straight to the rears as that was the pattern sensor and found? That the plug does not fit the cars wiring plug properly. F*CKING PATTERN PARTS!!!!!!!

The part where the clip you press to release the two parts doesn't clip home.
Replaced it temporarily with a spare e36 one I had and the light still didn't go out. Stumped me for about 30 mins until a quick search online gave me Stuarts answer above. All I've got to do now is order a BMW one in and fit it for her.
Oh, and avoid bl00dy GSF and ECP like the pox from now on!!!!!

B7's Motto. "If it's French, BURN IT!!!!!!"