Where can I find "ghosts"?

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

1. In the very beginning of the chapter, when the player approaches the window closest to the doorway, a flash of a figure goes past the the window from the inside. It is difficult to notice, and seems to correlate to the ringing sound the player can hear when looking in the window.
2. In the same lighthouse, there is a ghost at the top of the broken spiral staircase. The player can see it briefly at the top, and then it will disappear.
3. At the start of the journey go up the hill instead of following the beach. Then pass the first caved in path by walking on the ridge. Theres a small cave on top where the narrator will start talking about the hermit's cave. If the player looks into the cave, they should see a faint figure holding a flashing light, before it quickly disappears. This may or may not be morse code signalling the word "DAMASCUS". It's very hard to see apparently.

Buoy Chapter

4. Upon approaching the shipwreck, a figure can be faintly seen moving along the cliff pathway to the left of the wreck. It will then be seen standing at the end of this cliff pathway, though only when not directly looked at.
5. When approaching the bothy (house on the hill), there is a ghost that walks into the building as the player nears.
6. To the back of this bothy, behind it, there is a cave in the distance. If the player walks a little bit forward toward, looking into the unaccessable cave across the canyon, there is a figure holding a light that floats deeper into the cave and disappears.

Cave Chapter

7. Before entering the dream that occurs after plunging into water, there is a ghost that can be seen walking behind a waterfall. Very hard to see.

Beacon Chapter

8. There is a tall cliff with a candle lit on top of it with a ghostly figure standing beside it. This ghost can be seen from two different perspectives. The first s above and the second is on a path curving up and ascending this cliff after the paper boats. The player can faintly see the figure from a little distance away, but it will disappear like all of these ghosts when the player gets too close.
9. The player can walk along the beach, with cliffs and rocks to the right, and the water to the left. There are candles scattered along the way. There is one candle that has water underneath it near the area with the paper boats, and in this water puddle there is a ghostly reflection that can be one of three figures. It can be a hooded figure, a person with a deformed face/skull, or a transparent man or woman.
10. At the end of the game, when reaching the tower, there is a ghostly figure standing in the corner of the fenced in area. It also may be one of the same three figures as the reflection ghost in #9.

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).