Can a follower from Carcassonne City be placed in a completed, vacant area?
Yes. Whenever any city, road or cloister is completed, whether there is anyone in it or not, followers may be moved from the appropriate part of Carcassonne City to that area. Of course, in normal circumstances, an empty area is unlikely to be particularly valuable, but this option may serve as a useful way of getting followers out of Carcassonne City and back into your supply.