On the drivers door of my Audi B7 A4, there are 4 switches. Gas door, trunk release, towing tilt button. And at the bottom is a 4th button. It doesn't do anything. What is it there for?

i think the fourth one is if you have the interior alarm sensors. i think it adjusts it or turns it off completely. if you dont have that part of the alarm, maybe its just left blank. im not sure though