The rust gets a chance to take hold on cars not driven frequently. Not much you can do apart from making sure the pistons and sliders are free and removing the disks for inspection probably on an annual basis. Given the rears don't add much to overall braking unless you are towing or doing a lot of serious off road it's not somthing I'd worry about. 2012 TDV8 Vogue (L322)

I had my rear brake discs changed in Nov 2022, and the mechanic told me to use the Cruise Control more often on journeys, apparently when using Cruise Control it uses the rear brakes to maintain speed on downhill stretches, and I can confidently say after 3 years my discs are still in really good shape

Thanks both, now I know how easy it is to remove the pads and calipers i am going to start cleaning and lubing them every oil change. I change oil every 3 to 4k miles so this will keep my brakes in serviceable condition. 2012 4.4 TDV8 Vogue Santorini black ivory leather

Yeah, that’s what I do twice a year, remove the pads front & rear, clean everything and re-lube all the pins and pad top & bottom edges, 4 years later they’re still working fine.
Time for new discs/pads all round next spring. V6 Vogue - “Less is More” - less to go wrong!