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).

In Eve they’re called corporations/alliances, but it actually goes even deeper. More details here:

https://news.ycombinator.com/item?id=16048674

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.

Next YC batch to include:

leak-AB-ly: AB test your emails to see who is leaking.

Secures 10m in funding, with a valuation of 10bn.

Can’t make this shit up.

That is called a canary trap.

https://en.wikipedia.org/wiki/Canary_trap

Regarding the related technique, a "barium meal test", there is an xkcd:

https://xkcd.com/525/

I remember one lyrics website alternating normal and non-breaking space to prove a competitor was just copy pasting from their website

That competitor was Google, in case this is the incident you had in mind: https://www.digitaltrends.com/computing/genius-uses-morse-co...

This is something Elon Musk was rumored to have done, but ended up backfiring: https://www.autoblog.com/2009/03/05/not-at-all-quiet-on-the-...

Yeah, if you could just have VPs who know how things work, that would be great!