I would answer this question with the theory I have posted on this and other threads: one of the "guests" at the "party" excuses self to get snack from kitchen, or, perhaps, to use the bathroom. No one particularly misses said guest because they are otherwise occupied. He's been there before; goes upstairs, either carries out a sleeping child OR gets her to walk out with him. Might be Westerfield: the van Dams have lied quite a bit already, what would be surprising if they lied about knowing Westerfield? Or it might be another guest altogether. But it seems more and more like they came in and left through a regular door, not over that wall.
or was already dead when she left the VD property.