Mods/Admins join the game "vanished" (it's a game mod) as to allow us to, at any time, "vanish" from the other players, to let us catch them griefing, doing PvP, etc... It's super effective. I have seen a lot of griefers get caught this way, and i cant even tell you how many I have caught using this tool.
Yeah I have it so that when I am at home i generally only play at about 2 hours a day, as how i am watching my bandwidth usage, trying to keep it down. (It means that i wont have to pay for a ridicilous internet bill).
I normally stay invisible for about 30 minutes before i show up, so it is a good bet that when I do eventually "show up" that i have been online for the past half hour, watching players.