A few weeks ago (yes, weeks) my sister emailed to say she had a grinding noise when she braked (Ford Ka). Diagnosis simple, pads worn down to metal.
Today she finally got round to bringing me the car with new discs & pads to fit. And brake fluid. "It's losing fluid" she said. !!!!!!
When I took the wheel off I could see why - it had obviously pushed the piston out past the seal! How? I've never seen a brake disc like it. I measured the minimum thickness with a micrometer at just 4.35mm! Even bikes have thicker discs! I think she was damned lucky the thing didn't shatter.
Seeing is believing, so I've put a picture of the two removed discs at