Found it in the boot.
It's some kind of handle or a lip to open something, and it's rubbery (not hard plastic).

No idea where it came from...

 

Its the middle "handle" for the rear sun-blind. Because its rubber a good tug will pull it over the head of the screw. Good fix is simply to fit a screw with a larger head or be gentle when using the blind.

 

Thx!

 

Wow Alster that was a quick answer! I had no idea what it is, but I was going to make up something interesting and believable.

I would like a set of rear sunshades. I'm not so sure they were even offered in the USA. Where can I get them?

 

Please do, I'll pretend I didn't have read the above reply... Curious as to what you'll come up with :-)

 

Well I thought it could be an easy-grab front sun visor opener, as they are a bit stiff and you have to get your fingers over them to pull them down. That's a little too boring though.

You sure it isn't the headlight switch? Or the improved upgrade.

It could be a "Cool-Touch" door handle opener, given out at the dealers, so new owners could open the doors on earlier X300s with metal door handles (like mine) without burning their fingers! Those door handles get HOT when the car is parked in the sun! Not nice.

Or it could have been the "Stay-Put" adapter for the cup holder, so that it wouldn't let a cup fly out of it when driving! We really needed an invention like that.