The sort of biological nanorobots which could be responsible for a "grey goo incident" would pretty much *have* to be weaponized, wouldn't they?

I mean, they're intentionally designed to break other cells down into their component chemicals. So they must - by definition - be equipped with machinery to penetrate cellular defenses.  What creates a "grey goo incident" is these machines mutating to break down every cell, rather than just those (such as cancer cells) against which they were actually targeted.