Any car (but see below) that's sitting in humid weather will acquire a film of light rust. Usually, the film disappears with no harm after you've driven and used the brakes a while.

Once, several years ago, I let one of my cars (an old 1980 245) sit for a few weeks outside -- that was too long -- and really thick rust spots (in addition to the thin film) developed under the pads. It left a "stamped imprint" of the pads on the front rotors, and as a result, I got the pulsations typical of "warped rotor" syndrome, a feel that never went away. You could jack up the car and feel and hear as you manually rotated the tire so the rotor passed under the pads -- the rough feel and sound that it was severely rubbing. I had to either have the rotors cut or replaced to stop the really annoying brake pulsations. What a pain.

Nowadays, for the cars that I know I'll be leaving out of the garage for long periods (like my daughter's car), I put on all 4 rotors that are cadmium plated (from Eurosport Tuning & Brakes). They're expensive (I also get them slotted), but for example my daughter's car has been sitting outside all winter (with me just starting it up and rolling it a little bit about once a month) and there's not a single rust spot on the rotors. By the way, the plating covers the inside, where the parking brake shoes work, also, so you don't have to worry about that, either.

Regards,