I have washed my car many times and put it away, When I see it in the garage I can actually see the rust on the rotors. They are bare metal and when water hits them, they will rust up. When you drive the pads will rub the rust right away. I have never heard of any clearances to be so small that a slight coat of rust to cause locking of the wheel. I do not think this is normal at all. The braking system in that car is one that keeps aware of moisture and applies the brakes occasionally while driving to keep them dry, I think it begins this process if the wiper is on? but still something of this nature should never happen, there is a malfunction or you might have air in your hydralic lines causing this. You should call MBUSA. Good Luck!
Regards,
Brian