Took apart my rear brakes few weeks ago and I'm pretty confident that I found what is causing the squeaking that almost everyone on here experiencing at one point or another.
Long story short the brake shoes are rubbing against dust shield. The noise sounds JUST like worn out brake pads but it's not the pads. I don't have to tell you to search the forums to know that replacing the pads does not get rid of the noise.
You can actually hear the noise here in one of my videos.
I checked my pads and had over 1/8" of material left so it obviously wasn't that. Squaking also happened after standing for a but with e-brake on on a hill. Weird thing that it didn't go away or change if I started pulling the e-brake as I drove around. That gave me idea that it has something to do with e-brake shoes.
Taking apart the rear brakes:
To fix it all I did is filed the rust away and used heavy grease (brake lubricant works too) on contact points. Unfortunately I didn't take pictures after fixing it but it's pretty straightforward.
The car is absolutely quiet now after I did that.
After putting the pads back on you'll have to re-adjust e-brakes with the adjuster. I made it so that mine start gripping the brake after 4 clicks on e-brake lever.
I'm pretty confident that BMW techs are well aware of the issue but just being lazy about the whole thing. It's much easier for them to slap on a new set of pads and call it a day than try to take apart the rear e-brakes (which are paint to take apart and together).