Any caliper which is being naughty in the fashion that it won't fully release can be noticed during MOT, where the brake balance can be seen on a dial....however -
What I do is to jack both rear wheels off the ground, and spin them by hand......trying to feel if they spin with the same slight degree of resistance - they should both be equal.
Then, press the brake pedal hard for a few seconds, release and try again. The faulty caliper should now be dragging the offside wheel.
The garage doesn't have to honour brake pads and discs, as they claim they are wear and tear items....but in your case they should. Naturally enough, they will try to wriggle, and try to baffle you with bovine excrement science
Your question to them should be : 'How come only one side is worn, if the calipers are fine?' Let them wriggle out of that one!
I reiterate though - discs can turn brown with rust almost overnight, but after a few miles they should be clean again.
Hope this helps!
John