Not kept a journal per se, but I have created fake NPC records, both of real events in-game and as misinformation which party members have later found.
Once an Anti-Paladin (they did not know this was what he was, he disguised not only his identity as an NPC, but pretending he - the writer - was someone other than who he was) left behind a series of fake communications and records that led the party into a trap and ambush, as well as led them into a series of adventures designed to divert them from the NPC's purpose.
I have also developed codes and ciphers in Greek, Latin, and invented languages in order to conceal information and communications.
I have also created books of Myth, such as the books of the records of Pesh (in AD&D) which led party members form one set of adventure clues to another, such as to the Rod of Seven Parts or the underground ruins of the pre-Peshian civilization and the rotating Temple of Zereth. Of course back then I was a kid and could devote time to stuff like that.
It is an interesting idea though. And it could take a lot of different forms depending on what your intention is. You could be spying on the player party, gathering Intel, recording historical events, gathering bits of myth and folklore, writing down scriptures, pursuing your own agenda, trying to mislead others, recording magical or scientific experiments, putting your wirings in code, intentionally making copies to be found (or trying to keep everything secret), making sketches, detailing encounters with monsters, commenting on political affairs, detailing explorations of ruins, copying out languages and scripts, etc. Or all of these things, or more. And of course the NPC could start out by having found the lost journal of someone else, and adds his own content to something that already exists. Then that could be later tied in with the player party in some way.
