I'd check the area of fracture. If there is a crack in a piece of cast material, it will rust and/or corrode inside the crack until it weakens and suddenly fractures all the way through. The area of sudden fracture will be clean and show the granular structure. If the exposed surfaces are clean all the way through it usually indicates a sudden complete fracture.
One thing not mentioned or photographed is the rotors. A rotor could have come apart and done the same thing.
When you took the car apart, were there bolts in the threads that hold the calipers on? Are they stripped as if suddenly pulled out or did they come out still in the broken pieces?
