Keet said:We found an odd piece of plastic in a similar place in our e89. Turned out it was one of the screw covers in the door trim. Took us ages to figure it out.
Could it be #13 on this diagram?
https://www.realoem.com/bmw/enUS/showparts?id=LL32-EUR-04-2014-E89-BMW-Z4_20i&diagId=51_7275