That reminds me of all the espionage and counter-espionage that (at least used to) occurred in EVE clans/guilds (not sure if that's the right word). They even had custom plugins for their messaging boards where they could use BBCode to offer multiple words and the forum software would present a different combo of words so that if someone copy/pasted it then they could narrow it down either right away or with a few examples. As in they would have 4+ words in an announcement that had multiple variants and would record "We showed User A the following 1:1, 2:3, 3:2, 4:3..." (variable-word-instance-index:variant-index).
Also, I can’t seem to find a source for it but there’s also a technique used where screenshots have the logged in user’s ID encoded in the background of the screenshot by using hex colors that effectively look the same.
