It’s a British term for a one-time thing.
They think they sound smarter if they use UK slang.
I have an American expression for them that also involves the word off.
It's common in manufacturing, in the US. I learned and used the term in the 70's. Some items are mass produced, some are produced in small quantities, and some are literally one of a kind. Might be highly engineered one of kind, but still, it'd be a "one off," and all the costs associated with that one item have no further production to amortize costs against.
I think it's asinine to use the term in the context of criminal and terrorist acts. By that usage, Bonnie and Clyde, Valentine's Day massacre, the Bath, Michigan school bombing, Columbine, and on and on and on, are all "one offs," because none is the same as another.