You run a small batch of test product through with the balls in, then dump that product. They're used to calibrate metal detectors at the very end of the production line for detecting things like shavings metal coming off the machinery. It's likely that the batch wasn't dumped immediately, and was left unsupervised - so someone came along and packaged it because it wasn't in a container that was well marked as a test sample to be disposed of.

Food production safety can often be horrendously lax, because virtually all staff will skip or ignore parts of the production plan, either intentionally or unintentionally. Worse still, I've seen managers specifically write food safety plans that are designed to be cheated, for example, ensuring that a specific test sample is treated as it should be to meet all of the paperwork requirements, while all the food that will actually be eaten is handled at much lower standards.