Had never taken rear plate off car as dealer had originally bolted it on until yestreday when I installed a frame on the plate. This was dead center under the plate's position. I have no idea nor does anyone in any of the XF FB groups. I've seen one post here about it. Reply was guy turned it and front bumper feell off.

 

All of the cars have a hole there,some of the ones I have seen have a label next to it which is reputed to have the VIN and the paint code on it. There is also one under the bumper cover on the body shown in the following picture.

I said a hole not two holes, and the screw is inline with the OEM hole instead of the extra hole. Maybe your car has been in a collision and something is misaligned. I hope not but that could make sense.
The hole is an access hole for one of the bumper attachment bolts.