My father's Opel Astra F 2008 has thick bands of rust on the rear brake rotors. Both brake rotors are almost identical. Is it possible that the pads only cover the non-rusted part of the disk, or is this always indicative of a braking issue?
The inside band is not an issue. There is space between where the brake rides on the rotor (the shiny part) and the hat (the hub area). There has to be space here to allow the rotor to rotate independently of the brake pad and caliper.
The outside band may be more problematic. On most disk brake setups I've looked at, there will be a small band, usually measured in millimeters between the shiny part and the outside of the rotor. I cannot tell the exact measurement on yours, but it seems to be much broader than what I'm used to.
Two things to look for here to tell if you have a potential issue. First, look at the shiny area on the back side of the rotor. This area should mirror the same area you are seeing on the side we are looking at. Secondly, look at the disk opposite this one on the same axle. Does it look the same? If the rotors look the same on each side and on both sides of the axle, then there is probably not much to worry about. If, on the other hand, there are discrepancies, then there is probably a need to have them looked at. It could be a chunk of friction material has come lose and therefore there's not as large of an area of contact being made to the rotor. This causes uneven braking, which may cause other issues down the road.
If the size of the pad matches the "clean" surface then no, you don't have an issue - until, that is, you try to remove the pads...