I posted this theory a couple days ago. I put it out there that perhaps a woman who is a distant 'acquaintance' such as a neighbor, church member, grocery store worker, mail delivery woman, has been watching this family quietly from afar. She thinks that Haleigh's home is unfit, dangerous, abusive. She knows about Misti, that she leaves the kids home alone at night. She possibly does not have or cannot have children of her own. She keeps close tabs on this house and knows Misti will be out for the night. She slips in and tells Haleigh something along the lines of, "I'm Jane, I'm your dad's friend. He has to work late tonight and Misti can't be here. He asked me to come take care of you." And Haleigh, having had a revolving door of caretakers, doesn't find this to be strange and willingly goes with her. Now this random woman is caring for Haleigh somewhere far away.
This sounds out there, I know, but I keep coming back to this possibility.