Sorry to hear that. Hope you get it worked out (fixed).

If indeed it's someone else's paint on your car, a clay bar should clean it off. Unfortunately, consider the possibility that the black marks are actually the plastic underneath your paint.

What I prefer to do is: When I see an idiot in traffic, I stay the hell away - slow down, take another route, disappear.

In the meantime, think about investing in some 3M PPF on areas of your car. It sounds like you use your XF as a daily drive and so you're going to need some protection from the inevitable parking lot bashes.

What color is your car btw?