The CX5 has hill-start assist, which holds the footbrake on momentarily when you release the pedal. Pulling away in a manual (like mine), you'd normally have the handbrake on and not use the footbrake, but with an auto you'd have your foot on the brake pedal, so it could be that releasing? I live on a hill and having the hill start assist is useful when pulling away uphill, but a PITA when parking downhill behind something as you get a series of stop/start rolling judders rather than a smooth controlled stop.
It doesn't take long for a surface layer of rust to form on the disks when the car is standing, I've seen a film appear in less than 24 hours. It's probably that being rubbed off.
Alan