What is this nut?

[slk230red]

I backed my 2001 SLK230 out of the garage and this nut was on the floor about in the middle right side of the car. It has coarse threads and 3 indents to lock it in place, I guess it didn't work. The back side is recessed as if to fit a washer or something. I'll be jacking the car up tomorrow to take a look.
Maybe rear suspension, exhaust, etc.

Thanks.