Hi there Nethermore. Sometimes the Archivist spawns on the dunes outside, directly above the Archives where we are supposed to meet him. If you enter via the door as you normally would, walk down to the map itself then look upwards. You will see a large round hole or skylight in the ceiling of the room. You could try to climb up there if you think you can make it. Or mark the spot on the map then walk around and up to the top there.
I have noticed that his spawning is a bit erratic. However a number of people have now said that they tend to have a better chance of finding him if they enter via the roof, as opposed to the door. Just keep travelling away and back again, and he will eventually spawn in. If you have any further questions please just ask.