At the top of my PCM, is a non-functional "SOS" button. What is that black, same-shaped piece just to the right of it? Is it a blank for another option? A sensor? An indicator light for something?

My Carrera didn't come with a PCM manual.

 

I've wondered the same but here's a shot in the dark and maybe some one will correct me if I'm wrong, which I am quite often (
In Europe, there is the built in cell phone option which we do not have here.
So having said that, I assumed that it was a roadside assisatnce thing like "on star".
Can anyone out there confirm this?