the rear rotors are part of the hydraulic brake system. the rear parking drum brakes are part of a cable actuated brake system. the parking brake drums just happen to be located inside the hydraulic brake system rotors.
each system is independent and has it's own means of rusting while not being used. looking at the cable of the parking brake system won't tell you much about how the hydraulic system is working and vice versa.
the point of all this is that, in the rear of the vehicle, you have multiple systems that are subject to rust. calipers/pads could rust on the rotors and shoes/pads could rust inside the drums. if the rear wheels won't rotate, you'll need to check both systems.
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