Your squeakfrom the rear is probably not coming from the brake pads, it is most likely from the brake shoes.
On BMW's with disc brakes at the rear there are a second set of brakes.
If you look at the disc you will see the caliper, this is operated by the foot brake. Where the wheel bolts on there is a drum, within this there is a set of brake shoes operated by the parking brake, if you apply the parking brake as normal you will sometimes hear a squeak when you get in/out of the car, this indicates that the rear parking brake might be sticking.
You dealer should sort this for you.