After purchasing my dream manual dbs (2008), 2nd hand, several years ago, it was having a service at an Indy for the season and an MoT, it was discovered that just before delivery back to me, a grinding noise was heard.
This turned out to be the heads of the bolts, holding the bell to the discs, that had fallen off and were caught between the back plate.
Now the nuts on the outside facing were rusted but the actual bolt looked fine. It was the same for the inside facing heads - they actually showed almost no corrosion, so one assumed that the actual bolt was ok.
Where the heads had fallen off though, the corrosion of the bolt shaft was pretty dire, so much so it appeared fused and on closer inspection the discs had cracked along the bolt shaft, leading the technicians to believe that the expansion of the bolt due to corrosion had split the ceramic.
A few light taps with a piece of wood and more heads fell off.
This was apparent on several of the discs.
I highly expect the previous owners didn't take any notice of caring for their ceramic discs...but this is why you don't soak / jet wash your ceramics and then go and park it in a garage wet. Don't go and spray chemicals all over your wheels and discs...cover your discs or take them off your car if you want to wash them and park up. - I have taken this up with Aston and DVSA, but to no real avail. Aston said Brembo were going to release same brake care info in October 2019, but haven't seen anything to date.