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In Doctor Who Series 9, episode 3: Under the Lake,
The crew was evacuated to the faraday cage, and told the Doctor and Clara about the ghosts, who were currently trying to kill them. They tell us, that the ghosts come out only at night. The base switches back to day mode and the crew leaves the faraday cage, but then suddenly the base goes back to night mode.
We just have to assume that the ghosts did that, but how, since they can be present only by night mode?
As I recall, when O'Donnell switches the base back to night mode on the Doctor's orders so that they can catch the ghosts, they reappear suddenly and in position. I can't find this scene on Youtube to check, but the transcript bears out my recollection:
DOCTOR: We know nothing. We don't know what they want. That's what's getting us killed. Well, I won't run. Not any more. So, O'Donnell, kindly put the base back into night mode. We want to know what these ghosts are after? We ask them. We're going to do the impossible. We're going to capture a ghost.
[Mess hall]
(The lights go down in sequence in the modular corridors, then the night lights come on. Pritchard is half inside a wall, the alien is standing sideways halfway up a wall and Moran is hanging down from the ceiling.)
COMPUTER: Good evening. Entering night mode.-- Series 9 Episode 3, Under the Lake (emphasis mine)
Why did the ghosts appear suddenly in these awkward positions? If they actually materialised out of absolute nothingness when the base switched to night mode, it would make more sense for them to appear in, say, the places where they'd each died - or in the positions they'd been when the base last switched to day mode. But Pritchard was last seen in the middle of the mess hall brandishing a chair, not halfway through a wall, so neither of these possibilities can be true.
Presumably they appear in these positions because those are the positions they were already in. The evidence suggests that the ghosts have some sort of presence even during the day. They can't appear physically so that people can see them, nor can they attack or kill people, but they can move around. The next logical step is that perhaps they can have some sort of effect on their surroundings - for instance, changing the base from day mode into night mode so as to give themselves the ability to manifest visibly and attack/harvest more people.
