Yep, I understand your thinking but I've just been out to triple check and that mark you have circled isn't a mark, it's a reflection from the camera flash, the plastic there hasn't been twisted, damaged or distorted in any way. I know that looking at that photo there is a hole with what looks like a washer mark around it (down and to the right of the green circle) but I'm pretty sure it wasn't previously attached to another part of the undertray. Whilst I know it's a bit staggering that none of the 3 VW guys who looked at it the other day knew what it was doing, the one thing they all agreed on was that it wasn't loose and none of them thought it had moved down from anywhere (ie. they were unanimous in thinking that it was either there for a purpose or had been incorrectly assembled).