Front Disks Rusting

[Bad Moon]

I've noticed that after I wash my bike the front disks will get some surface rust where the pads are (after sitting over night). This makes the pads stick to the disk and I have to rock the bike to break them free. I've had this happen at least 3 or 4 times. The last two times I even rode the bike a little (a couple of miles) after washing it. The most recent time I even drug the brakes a bit to get them hot, thinking it would evaporate all the water (it didn't).
It's not something I'm concerned about but I've never had this issue on a bike before and was curious if anyone else has the same problem.