Estate - What is this rubber?

[Hylen]

Was fitting mud flaps on the Estate and on the last one at the passenger side rear wheel one. After popping out the studs and pulling out the rear bumper to put in the nut springs, a rubber fell down that I did not see on the other wheels.

Not sure where this goes back in, could not clearly see underneath the car.

In the middle it has written on it BP 1/2

 

[APS]

That is a normal bodywork plug/grommet. It would fit in a hole somewhere in the body to seal it. It looks entirely clean, so it is probably not an external facing one and thus may not matter too much if it is installed or not. Take another look underneath, behind the bumper to see if you can find a suitably sized hole. It could also just be one that was somehow dropped and lost during the assembly of the car.