Sounds like dirty pistons on the brake caliper. The dirt wont let the pads retract just that extra bit. The pistons stay exposed long enough to get a dirt ring around them while they are exposed due to the width of the worn pad. The way to correct it is to remove pads carefully extend piston, they will pop out if allowed to travel too far. Clean with a toothbrush and some brake fluid. Tho other option is to live with it.
To be sure you do not have a bigger problem is to ride the bike and without using the rear brake to stop and feel the rear rotor to make sure its not blistering hot....