A hidden outpost is an outpost which does not show up on the map and cannot be accessed through map travel. Hidden outposts are generally accessed through NPCs, although the Factions elite mission outposts can be accessed with appropriate Passage Scrolls.
Additionally, some hidden outposts are used by the developers to move quickly to a specific part of the game for testing purposes, and meant to be inaccessible.
One hidden outpost ("The Ebony Citadel of Mallyx"), a visual copy of Gate of Torment, was discovered by players through an alleged client hack, claimed by ArenaNet in January 2008. This outpost made it possible to go directly to the The Ebony Citadel of Mallyx without solving the four preceding quests. A number of players used this outpost to gather and sell Armbraces of Truth and were banned with the release of the 10 January 2008 game update, which also locked this outpost, preventing further players from reaching it by mistake.