Great trip fellas!
The story I heard on the mystery bunker: Some guy from Grants Pass area bought 300+ acres out there and erected the building. He apparently owns a sand and gravel or heavy equipment operation and had all the gear to build his hut. During construction we saw quite a bit of equipment out there with several shipping containers. Have camped on the north side of the butte many times and was kinda bummed to see his structure now permanently identifiable on the horizon over there but at least he tried to color it sage.