The corridor beyond the door is very dark. The boar can see much beyond the light spilling into the corridor from the light of the torch one of the NPCs is carrying. With its low-light vision, it can see roughly 80 feet in either direction (the second 40 feet is shadowy illumination for the boar) from its spot on the map.
Thus, as per the battle map below, the boar can clearly see the cultist standing 30 ft. away from him, but the other two cultists are within shadowy illumination. Thus, they have concealment vs. the boar. The boar can clearly hear the two men to the south, but it doesn't hear the cultist to the north.
Once Caerth moves into the hallway, he can see all three men clearly and can note the other features marked on the map below. The two cultists to the south are running away from the broken door. There is an open corridor/passageway to the east of Cultist #2. The cultist to the north seems to be standing guard, but trying to stay quiet and hidden. Caerth can see the cultist clearly with his darkvision.
Cultist #3 - Initiative: 1D20+1 = [8]+1 = 9