The Sentries were an organization that existed within the Iconian civilization. This agency was formed by the remnants of the Iconian Empire after they had fled the devastation of their home world when it was orbitally bombarded by races that feared them due to their gateway technology. After departing known space, the Iconians formed a body of their people who gathered when their equipment was in use. Under their laws, they were required to monitor the use, record the species that employed the gateways and watch them for signs of incursion. These rules that governed them told them to simply watch and remain vigilant in case they were threatened which they obeyed. (TNG short story: "The Other Side")