One of my favorite HN comments of all time [0] suggested that MMO's provide plentiful means of covertly communicating:
>MMO's are packed with possible communication channels in addition to chat. Ever wonder if that annoying gnome in the auction hall is jumping in morse code? Could signals be sent with bids? Could a character's inventory contents be arranged to leave a message to someone else who shares the login info? Is that nonsense coming from what you presume to be a bot-controlled gold-farming crew really nonsense? When a game goes to great lengths to simulate a world, the possibilities for covert communication are nearly limitless!