Relax .... it's not a serious issue. Irritating I know, but all cars with disc brakes, front or back, will do this, regardless of the brand of pad, grade of steel, etc. Think of it as a design fault. Mr disc-brake-designer either lived in California where it doesn't rain, they don't put salt on the roads, and rust is a shade of brown rather than a corrosion problem or he thought that this occasional binding due to corrosion adhesion was a small price to pay for a better braking system.
There are a few things you can do to minimise the problem:
If you've just washed the car, move it once before you park it - it only needs to be a few feet, and then put it away 10 minutes later or so. This two stage movement helps with draining, drying, etc.
Because your car is so new, the pads haven't bedded in yet - particularly the rears which don't have a great deal of work to do. (I renewed front pads at 50K, rears at 95K) Once they're really smooth & shiny rust won't take hold so readily. It will tend to be more superficial and you won't get so much of a clunk when it frees off. A few good handbrake stops will help bedding in, but all the usual safety caveats apply.
Finally, as the dealer says if you've garaged it wet, leave it in gear, rather than on the handbrake. That will prevent it sticking.
