snagitseven

I used to believe that too. I've been proven wrong as the rears usually do wear out faster than the fronts on the A6. This is true especially with ACC and the Presense Braking Guard which mostly use the rear brakes to slow or stop.

tenspeed

There isn't any place that says which pad is worn. The sensors are on the inside pads, front and back, drivers side. In my case, the rear pads were worn the most. I'm the second owner and rarely use cruise control so I expect my front pads to be worn more at the next service.

My new pads were 13 mm thick and the sensor comes on at ~3 mm so you have some time to decide what to do.

This is the gap on the front caliper with new pads.