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.
Best Answer
The wikia lists 10 and has screencaptures of the locations, this thread corroborates all 10 of them, and this thread has the same 10, so assembled, this is all 10 that are reported to have been found:
Lighthouse Chapter
Buoy Chapter
Cave Chapter
Beacon Chapter
A lot of them are very faint and hard to see, as the centre of the screen comes closer to some of them, they will often vanish and as the player approaches them, they will vanish as well. Since it's hard to say for certain that there are no more, I can't honestly guarantee that this list is complete, but it is fairly thorough and these are all the reported ones to date. An excellent video walkthrough of all 10 of them can be seen here (number 10 in the video is the other perspective of the #8 that I listed).