Ahahahahahah, oh man, I'm so sorry she startled you. This is so funny, because it's such a harmless feature, but I get why you're worried by it. It's okay! ! She's just visiting. I'm in the same situation on my newer file too - Jitters won't go away, and he's like... Have you
seen Jitters?
He creeps me out on a normal basis, but having him hang around at weird hours of the night, all by his lonesome. Yikes.
No, in all seriousness, this is just a fun way for you to remember the villagers who've moved out of your town. You can have some fun new conversations with them that you wouldn't have had when they were living in town! In fact, this little feature caused my favorite thing that's happened with my game, ever.
In my older file, I had Chief and Lucky move out of town around the same time, due to a time-traveling accident. I had jumped forward a few months thinking I had timed it right so no one would move, and before I knew it, both Lucky and Chief had moved out in the same jump. Here's the neat thing -- when one was on Main Street, the other would always be there too. If Chief was hanging out, you could find Lucky in the shops, and vice versa, 100% of the time. I think the game got them linked somehow -- because they moved within the same time frame, with no specified date, they're always together. They were best friends when I had them in my town, so I like to think they moved out and they're now roommates, and when they come shopping, they've both come to visit me!
You can also use this to your advantage in cycling towns. When a villager stops showing up on your mainstreet, then it's been 16 villagers since they've moved out, and they've forgotten about your town. So they're available to move back in!
Margie will have some company soon! In the mean time, just say hi to her now and then so she doesn't get lonely.