The key stores the current CBS service interval -- I don't think it stores any particular data on how you drive, etc.

 

The brake pad sensor is not fiber optic. The idea is that if the bake pad wears down to the end of it's life, the brake sensor will wear off some plastic on the sensor and make contact with the brake rotor. That completes a circuit and lights up the "replace me" light. While I haven't inspected a BMW sensor beyond looking at a couple pictures, this is how my Benz did it. I don't think the brake pad sensor has any role in estimating pad life - all it does is tell you "we're done". The estimating is done by other things, like driving style, highway miles, etc. A Toyota and Nissan I had used a "noise" sensor. It was a little metal clip on the end of the brake pad that would make contact with the rotor and make an awful noise. They figure that's enough for someone to get concerned and take the car in.