If I were to write one of those law and order episodes "torn from the headlines", the open door would either be a smokescreen by the parent(s) trying to cover up the fact that they sold the kid, or something equally diabolical and disgusting; or I would have the neighbor unlock the door when he was there for the pizza and ???, then return later to take the girl without the parents knowledge.
The truth could be close to either one of those scenarios, or far from it. I just hope this doesn't turn into another Jon Benet Ramsey thing, with nobody ever being proven guilty for whatever has happened to this little girl.