I can guarantee the brakes didnt wear out @ 13k miles. What happened was the CBS data said the brakes are due when they really were not and the warning showed in the car. This is very common for F10s to give a false brake warning saying that the brakes are due prematurely. So the OP probably got new rear brakes @ 13k but for sure did not need them.
s5snake5. In colder climates it is normal to get a little squeeking from the brakes. Especially on the initial drive after sitting. The rotors may develop a slight haze of rust on the rotor from sitting and thats what is causing the squeak until that rust goes away after driving for a short time. Just like when you wash your car you get a slight haze of rust on the rotors. I wouldn't worry about it.
2013 Alpine White F10 535i. JB4 S2, Quad Tip Magnaflow Exhaust, H&R Springs
2011 335ix Le Mans Blue w/ M Sport SOLD
2003 Turbo Cobra 765rwhp/650rwtq 9.77@126 pump gas, lifting @ the 1000ft mark