This has happened to mine and looking around the forum I've seen it mentioned a few times. It might be worth adding to the buying guide.
On a car with about 60,000 miles the rear brakes pads may have started to break up. It looks fine when inspecting the pads through the calliper but when you remove the pads you see the problem.
The inner pad also wears more than the outer pad and may be down to the metal. The inside face of the rear brake disc is also not visible, it can be rotting when the outside face looks fine.