I thought they had metal detectors to scan for such things!...............
I've worked in various food factories, although never cheese....
They do have metal detectors. It could be as simple as a line worker being responsible for keeping a record of metal detector function didn't record it properly over a shift. Or it could be QA finding a small nick or small piece missing from a stainless steel production line, that somehow wasn't found in metal detection.
I'd say food factories are very careful about such things. The big corporations like Walmart, Coca Cola, McDonalds will make a supplier's life absolute hell if someone gets sick from a product they sold.