Before you top up check your Pads for wear because as they go down the fluid will also fall. Never needed to top mine up with fluid at all.
As for the rear discs I presume you have a PP with the bigger Front Discs so the rears are hardly used and the faces never get use to clean them properly. The inside of the disc on mine was much worse than the outside but VW say it is normal.
I think the rear brakes are used by the adaptice cruise control when slowing down more rapidly than just coasting, so ACC use should keep the rear disks clean.