The location of Outpost 2
Outpost 2 was an outpost constructed on an asteroid by Starfleet in 2160. This outpost was one of seven Earth Outpost Stations located in Sector Z-6 on the Federation side of the Romulan Neutral Zone.
In 2266, the Federation lost contact with the base. It was determined, by the USS Enterprise, that the station was destroyed by an unknown vessel that was later discovered to be a Romulan Bird-of-Prey.
During the Neutral Zone Incursion, the location of Outpost 2 was labeled on a map of Sector Z-6 that was displayed on the bridge viewscreen of the USS Enterprise. (TOS: "Balance of Terror", production art)
Between 2266 and 2366, this outpost had been rebuilt. In 2366, Outpost 2 was warned by Starfleet about an alleged Romulan base on Nelvana III. (TNG: "The Defector")