I bet they will find some kind of pouch, pendant, ring, hair clip or other small accessory within a gap of the couch and Mina is actually bound to that item.
If she goes too far away from the item she will "teleport" back to it.
As for why she can freely move within the house, the item is probably counted as part of the inventory of the house and as such the house is like an "extension" of the item allowing her to move freely there.
Something like that.
@Aesthia:
You are a bit late to notice the issue. 😅
How can a ghost eat normal food to begin with?
Even if we say a ghost can eat by transforming the "eaten" food into spirit energy (or whatever) then how can the ghost gain weight?
🙃