Ponyville is a quiet little village. If you didn't know any better, you might say that it never changes.
Meadow, however, knows that this is not the case. Ponies change almost constantly. You can catch it, too, just out of the corner of your eye. Meadow is an expert at finding things. It even says so in a snarky little footnote on her resume.
Once you've seen something that you shouldn't, it's hard to avoid looking at it again. Once you've seen something unexplainable, you can't really go back.

And sometimes, once you've seen it, you might go missing.