During my local government career, we once had a situation where somebody was leaking confidential items. I was given the task of making every copy of every paper unique, by omitting a full stop at the end of a paragraph, or inserting a double-space between words, always on the front page, and keeping copious records of who received each copy, so that if we ever saw a copy in the wrong hands, we could identify who had shared their papers. Only my line manager and I knew this was being done. It worked!