They're just stopping by for a visit. Nothing more complex than that. It's just a nice little quirk of the game that your old villagers will still come visit your town.
I have 2 games and the villagers move from one town to the other. Fuchsia moved out of my second town (after moving in from my first town) and has shown up on Main Street in the first town. Camofrog moved from my first town and is still living in my second town, but he shows up on Main Street in the first town also.
- - - Post Merge - - -
And yes they show up in the shops and Club LOL also.
The only two villagers I sent to the void are Ava and Camofrog. If I see Camofrog I'm going to beat him with my net. Ava would be nice to see though c:
Unfortunately, they seem to hang out on Main Street for weeks, if not longer. I see Harry and Yuka there just about every day. I wanted them gone for a reason, and they came back to haunt me!