Fans run on diesels when they are carrying out a regen of the DPF. Not 100% certain if your car has one but 2008 is on or about the time they were fitted.

Other than that it could simply be the car is running hot. On our old TDi the fan would cut in when we were in traffic yet the temp gauge would still be showing normal. Was told temp gauges do not move until its virtually too late thus mean nothing on a modern car.