Hi Paul,
My first XF, a 2010 model, was fitted with a detachable tow bar that was inserted horizontally. When not in use, the mount and the electrical socket were visible.
On my 2016 XF, a Witter detachable is fitted, but the tow bar mounts vertically from below, and the electrical socket swings up behind the rear valence.
When not in use, there is no indication that a tow bar is fitted, and the lines of the car are not spoilt. The only downside to this towbar , is that it cost £650 fitted.
I hope this is of some use to you.