This happens every time with the Plus 8: the discs rust and the pads stick to the disks.
As has been suggested, a little gentle use of power, rocking the car, preferably with so that the car is being driven towards the exit and not the wall...!
And yes, never leave the handbrake on when the brakes are wet. As advised, leave the car in gear, again in either reverse or first, depending on which way you park the car in the garage.
