Despawning

From HGWiki

When all Player Characters leave an area, any Mob that had been spawned in that area is automatically removed - "despawned." This is sometimes used to clear areas of mobs very quickly by spawning them (usually in GS) and then leaving the area, so that the now empty area can be passed through safely by other party members. In Legendary level areas, despawning any mobs usually has the effect that any loot and remaining spawns in the area are despawned as well, which may lock up a run by removing required quest items (see "anchoring").
Note that there is a despawn timer which is started when all player characters exit an area, if you reenter before this timer has expired you will find all Mobs still present.