When I first started driving in 1974, you had to check the thickness of the friction material on your car's brake pads regularly. If you did not, it would wear down to the backing plate. This caused metal to metal contact and ruined the disk.
In the 1980's, manufacturers put two wires in the friction material. Once the minimum recommended thickness was reached, the metal of the disk created an electrical connection between the two wires, switching on a dashboard warning light.
In my wife's 2004 Forester, Subaru adopted a much simpler mechanism. They attached a thin strip of spring steel to the end of one brake pad. Once the friction material has reached its minimum recommended thickness, this steel strip contacts the disk. It does no damage but causes a very annoying squeak, telling you that the brake pads need to be changed.
This first happened after the vehicle had done somewhere between 40,000 and 50,000 miles. I changed the pads and the noise stopped. It happened again somewhere between 90,000 and 100,000 miles. On that occasion I changed the pads and disks. The vehicle has now covered 142,000 miles and the noise has just returned so I decided to take a couple of photos. The first shows the two brake pads from the left hand side of the vehicle. The pad at the top is shown side on with the friction material to the top of the picture. The spring steel strip is attached to the left hand end:
The second photo shows the spring steel strip in more detail. You can see that the end of it is in line with the exposed surface of the friction material, which only has a couple of millimetres of wear remaining:
15th July 2017: Shared with Subaru Forester Owners (Facebook group).