You don't even need active measures. If a publisher is serious about tracing traitors there are algorithms for that (which are used by streamers to trace pirates). It's called "Traitor Tracing" in the literature. The idea is to embed watermarks following a specific pattern that would point to a traitor or even a coalition of traitors acting in concert.

It would be challenging to do with text, but is certainly doable with images - and articles contain those.