Brake pad sensor
On the older BMW's that I have worked on, as the brake pad wears down the rotor contacts the wear sensor. As the rotor contacts the wear sensor it removes material from the wear sensor and eventually breaks the wire that is molded into the wear sensor. When the wire breaks the circuit is opened and the brake pad warning light illuminates. I have outened the annoying warning light by cutting the wires to the brake sensor and splicing them together, thus closing the circuit. Maybe the pad monitoring method has changed on newer BMW's. I resused the same sensors when I put different pads on my 2009.