Quest: Dawning of an Endless Night
Location: Airport of Solomon Island (Kingsmouth, Savage Coast, Blue Mountain, ...)
After watching a cut-scene in the Fringe Tunnels, you have to go there to find what Beaumont was looking for.
The object is inside the airport, but NOT the part controlled by the Orachi. So you don't have to enter the Orachi restricted area.
You can walk around the "1.5 skulls" mobs and jump above the fence.
The object is a plan on a table inside a hangar. The door of the hangar is on its side and it's open.
You do NOT have to explore the tunnels with mobs or electrified water.
The quest markers are quite off and incoherent.