Mystery Mercedes prototype

This picture was taken at a Mercedes facility, as a 2014 S-Class prototype was coming back from a test drive.

The left car in the background is another 2014 S-Class prototype.
But what is the car next to it?
At first it does look like another S-Class. But the headlights are higher. Side windows more angled.
And why is it still fully covered?

Anyone care to guess what this is?