From the description, slightly corroded rear dics. The answer is use and firm braking.
Handbrake use leaves small rust deposits on the rear discs that should be worn off in normal use. If you're a low mileage user or don't brake much, you may not be clearing off the rust residue. That is of course if the pads are not worn below say 3mm or less. In which case, replacement should cure the problem.
FWIW, every time I take my car out, I make sure from a speed of >40mph, it gets a proper firm brake to keep the brakes in order and working well.
If you park your car off road and on the level, then leave the handbrake off and just put the car in gear to help avoid possible issue.