I found some old notes on this on my wiki, and figured someone might find them helpful
There is a known issue with the Citroen C5, where the rear calipers may end up rubbing against the inside of the wheel (rear caliper corrosion syndrom).
Basically the issue is one of water ingress. Corrosion develops between the mating faces where the caliper is mounted to the suspension leg, and the caliper is slowly pushed out at an angle, ultimately causing it to rub against the wheel.
It's identifiable by metal being worn away from the caliper and/or the inside of the wheel.