I'm having similar problems with figuring out how to implement special game events such as entering a specific sector playing a cut scene, or having gates bringing players to a completely new sector; should these 2 things be represented as an entity?
If your sector has a collider component, when new entities collide with it (aka enters the sector), the collision system informs other systems of said event. Your CutSceneSystem may be interested in collisions between Sectors and Entities and if such a collision event is raised, it gets the current sector id/name, locates the cutscene files and begins their playback for the user experience. In this case, its a combination of the Physics System and a Game System interacting by way of events/messages/callbacks.
As for gates taking players to new sectors, this again is basically a collider component in the ECS that raises a collision event that causes your SectorTeleportSystem to fire. It begins by playing a wormhole warp cutscene sequence while the new sector is loaded from disk. After loading is complete the cutscene sequence ends and you find yourself in the new sector.
As phil_t said, some systems can respond to game world events/triggers but are not necessarily manipulating entities. The events/triggers they do react to however can very much be something that the entity component system triggered based on some component system that manages a particular game aspect.