All of these be like "paranormal" ideas or normal ideas to add.
No.1; Mannequins
My first idea is there are random Mannequins standing around the map. Might sound weird but Halloween themed?
No.2; Paranormal activity
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There can be random doors opening around the map in front of your eyes, and in my opinion, if your the last kid and the door opens in front of you the first couple of times I will be spooked and run away.
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Lights can flicker randomly very quickly plunging the room into darkness and out, but it will stop after a while. But it will turn on in the end.
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I do not know about this one but there can be blurry ghost mannequin figures (Can be representing the dead kids) that can be in a room. This only appear after kids are caught and it will be blurry figures of the kid mannequins. They will appear not for long before turning into mist.
Idea-2
If you grab a camera and go to where a kid has been caught, you can see their mannequin standing in the spot. -
Paintings; Some painting will randomly start moving attracting the attention of the person near it.
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Random emote calling, randomly around the map you can hear random fading emotes which sound distorted, this happens when your the last kid
(Updated because of @paper steppe message idea, thanks for the better understanding Enzo)
@rose robin (Cool recommended idea)
When a kid gets caught, the kids can choose to haunt as a ghost. They will appear as a mannequin but in a light blue glowing shade like a ghost and will first be seen on the spot where you died.
They can fly through walls and up but they can't interact with any item.
Plus they can't see any keys of usable items so they can't help the alive.
I'm not sure about all of these but just a quick "Paranormal touch, I will add more and you guys can recommend them and I can add them.

