Its the end thingy off of one of the front (dont know whether the rear ones are the same) drain tubes for the sunroof. Both mine have dropped off and I keep them in the car ready for when I have the wheel arch liners out in the future so I can put them back on but to be honest I havent had any ill effects by not putting them back on.

Where the drain tubes exit they are pretty high up and well protected by the front wing and wheel arch liner so I think they were just a bit overkill anyway. I shall still be putting mine back on when I next have to take things to pieces in that area.

-Wills

had a look at replacing it last night, fook me, how the hell are you suppose to get anywhere near the end of that tube.

I will do as Wills, it can stay in the car until the day I have to strip the whole front corner off