I think I was being haunted the other night. While playing the Archive mission, I found a Ghost up on the branches of a tree in the N/Gen branch, before you get to the actual Archive itself. For some reason, I had never thought of looking in the branches of trees before. Another mission or two on Venus and a Patrol on Mars later, I had about six or seven Ghosts in my arms, either from trees or just looking up all the time.

At one point, I was hopping up and down on some branches declaring there was nothing there when Ragashingo told me to look down. There was one in the roots!

Obviously you can go look up their locations, but I've been just keeping an eye out while I've been playing and only have a few left now to find. Thought it was interesting. (If not, ignore me, move along, nothing to see here.)