I agree with Peter, that looks like steel to me.
Its shape suggests it's part of a tooth so hopefully
the remainder is still intact.
An unusual event Neil, but is it worth
dealing with unless you've got another reason to remove the box?
I'm not sure.
Not wishing to scare, but if there's more debris like this floating around......
I suggest you fit a magnetic drain plug if you don't have one already, and monitor
closely.
Nigel
