This is an interior trim piece.

As you know, the rear seats of your X5 have a reclining adjustment that allows the seats to recline as much as in one of those turboprop commuter planes. In other words, it is a worthless feature added to "me too" Mercedes.

The catch for the seat goes back and forth on a little electric motor controlled by a switch on the overhead console or in the rear of the car.

This piece you found is a bezel that covers the seat catch. There is one on each side, this appears to be the passenger side.