[Fenring]

I ride often in rain, and sometimes wash the bike too and leave it wet to dryout on it's own. It's normal for rust to form on the clear surface of rotors, especially under the break pads where water lingers, so these stainless steel rotors come as surprise to me. I've never seen rust form on the edges or near and around the bolts though. Could be salty air if you ride near the sea, or maybe you washed the bike with an over agressive detergent?

EDIT:
This morning I discovered similar rust stains on both brake rotors. The bike was outside over night, because i couldn't use the garage while the floor is being tiled. The night was cold, foggy and there was little rainfall as well, non directly on the bike, but enough to create a moist environment.