In case anyone was wondering, the garage has indeed identified the oil cooler as the source of the problem. Grainy sand filled the space within the water jacket and all the pipes that had been inserted into it when we removed it. Plenty, not just a little. The result is that the cooler has burst due to the buildup of pressure.
From what I can see, it has nothing to do with the engine and is neither magnetic nor metallic. I am familiar with the car's history up to the last year's 30,000 miles, during which time it has had the oil changed and a serpentine belt installed, but other than that, it has been completely unaltered. Nothing has been done with the coolant system.
Is there anybody who can guess what it could be? Although it's strange that it went 30,000 miles before exhibiting difficulties, one reasonable hypothesis is that a radweld similar to Bars Leak may have been added (too many times) prior to my ownership. My only other theory is that someone or something was deliberately added to the coolant system, but nobody other than people I trust has been near the vehicle.
Is anyone else thinking about possible other explanations? I am grateful.