sorry to dig up an old one but i eventually found out what this was when i had an extra hand to operate the handbrake while i took a looky. basically all that was causing it was the lever on the caliper which the cable links to was sticking, so when you released the handbrake it wasnt ratcheting to give any adjustment, so as the pads worn the caliper wasnt adjusting to compensate this.
a little wd40 to free it up while you work the mechanism with a small pry bar and then so white grease to keep it fresh is all it needs!