I first noticed this when I was playing through "The Beacon," the last area. I looked up on the cliffside while walking along the beach, and was surprised to see what looked like a person looking down!
I thought perhaps I was mistaken about it being a ghost/person and that it was just another standing stone, however, when I arrived at the location along the path, there was no stone to be found...
After loading up the second area, "The Bouy"[sic], I then noticed this:
This one actually moved. When I first noticed it, it was running along the path where the arrow is, then stopped at the location I took the screenshot.
After standing there by the boat a few moments, I noticed something. The opacity of the "ghost" changes depending on where you were focusing your camera. The arrows show the distance from the edge of the camera.
I outlined it in this picture on the right because it's very, very faint.
Where else in the game can I find these "ghosts"? I'm looking for a full list of potential locations in one answer, please, not one-answer-per-location.