Overhauled the rear calipers today and found that I only have a wear sensor on the driver side rear caliper, nothing on the passenger side.
Plus the two up front for a total of three, does this sound right?
Also found the most disgusting thick green muck inside the caliper piston chamber, very satisfying to clean out and re-build.
Chris

i thought it was two...

Ditto, 2 sensors - left front and right rear.

http://www.stormpages.com/countchocula1/bmw/bmwetm133.jpg

So it is, my screw up.

Were you looking at the ABS sensors?