"I found this blue disc in a packet of sour cream crisps. Its has the words “ferrous 25mm BST, cert number 213026B” on it."
A: "It’s a testing chip. It goes through the metal detectors to ensure they’re working. There’s a problem here though. You send X number of testers you get X number back. If you get X-1 or some other number you stop the line until you find your chip. The entire purpose of the test is to make sure that stuff like this–which is supposed to simulate a foreign object–does not get through. There’s usually paperwork to document this. Write to the outfit and tell them what you found and rest assured there will be a s@#tstorm on the other end."
Where do you call it like that? I only know it as Schultüte.