In classic ragnarok online there is in game GM characters...
Speaking of old RO, when the GM logged into the game with a character and made themselves invisible, the game client would just skip rendering their sprite. Packets announcing their movement and so on were still sent to the client, and this was how bots would detect the presence of a GM and teleport away. If you were around a group of bots and they all teleported away at the same time, an invisible GM had just walked onto your screen. The point of this was supposed to be to avoid getting banned (which the GM could do by right-clicking on you) but in practice the GMs would ban people from an interface outside of the game client that accessed the database directly because it was less of a hassle.
Another effect of this was that the invisible GM still took up space in the world. You couldn't stand on the same tile as them. If you found an odd square nobody could stand on in a busy area or a strange gap in the middle of a crowd, there was a good chance that a GM was standing there.
At one point iRO-GM made a joke about walking through Prontera and watching all of the merchants teleport away like a wave. Using the Kore bot as a vending bot was very common at the time, and most neglected to disable the "teleport when spotting a GM" option. (They also usually didn't bother to change the bot's default password, so I could PM them, authenticate as a bot admin, and then change their account password to something rude and log them out...) I explained the movement packets and demonstrated how I could stand next to her but not on top of her while she was invisible. She then demanded to know exactly how and why I knew that... I think she thought I was writing a bot. I had to explain that no, I was just such a nerd I watched internet packets for fun.