Hi guys...one more question....

I have a 2006 330i BMW about 160K miles, I have had a brake inspection and the front brake and rotors and sensor were replaced....the brake light is still on as well as the red picture of the car on the lift....the mechanic says that my rear sensor needs to be replaced and that my brakes are fine....but I notice that occasionally my pedal is really hard upon stopping and the car seems to slide a bit....what causes this and why did the mechanic miss this?