They do get a light coating of rust very quickly these days. I've noticed it since they stopped putting asbestos in the pads. Don't know if that is a factor or not. It'll disappear the first time you put the brakes on and reappear quickly if parked for a day when it's been wet.
If I dont use my car for a few days and leave it out on the drive the disc soon go rusty, and they are three years old, never had any problems, guess its just a case of metal and moisture , rain or damp making them go rusty looking.soon goes after driving around for a while.
Brake discs are generally made from cast iron and cast iron rusts. It's called oxidation. You can get discs made from stainless steel but they are more expensive.
yes as chuck says if they are clean after you've used them then that's fine, if not that means that the caliper isn't working and it will just get worse so if that's the case get it sorted. Quite normal if a car is left standing for a few days.