This is almost certainly just rusty rotors stuck to the pads after sitting for a while, especially if they've been wet or have salt on them from driving. The first time you move, it breaks free (this is the clunk) and the remaining rust is essentially wiped away by the rest of the drive.
You will especially experience this is a garage, because it allows the salty water to be a liquid (and rust far easier) instead of freezing outside.
Most cars do this. Nothing to be concerned about.