Front tyre rubbing wheel arch?

Having been to get new front tyres yesterday (don't start me on the muppets at the tyre place!), I took the opportunity to have a good look at the brakes/suspension/other front bits while the wheels were off.

Anyway...I noticed that there was a round black mark on the front of the N/S wheelarch, almost as if the tyre had contacted the arch at some point. The tracking is fine (doesn't pull, even tyre wear), 205/55R16s, original wheels & tyres. I don't think the front suspension gets too much of a hammering, at least not as much as the rear.

Has anyone else found this, and did you ever find the cause? Its a 2.0TDI Elegance Estate BTW.