Though I mainly play the game for the scoring aspect, I do keep an eye on what state my Nightopians are in by checking the Nightopian Collection in Christmas NiGHTS every so often, and one odd thing I've noticed recently is that the Collection says I have only one Nightopian in Mystic Forest, despite the fact that there's clearly three or more in the level when I play it. I thought this might be a bug until I went back to Frozen Bell recently, and saw all those Nightopians waiting at the train station - Nightopians which can't be interacted with and which don't show up in the Nightopian Collection.
Now the Nightopians in my Mystic Forest mostly only appear in the background (i.e. they're non-interactive) too, so I'm pretty sure that they're also just level "props" rather than real Nightopians, though not as blatant as the Frozen Bell ones; they always hang around the big temple structure that you can go inside. But am I wrong about this? Has anyone else noticed this sort of thing, and does it happen on any other levels?