Wait, hold on a second...
It was the mother of a child that told them Sandra said she was going to run away? I thought it was originally "a five year-old playmate." I don't have the link, but I remember that from the first day or two of the disappearance.
First of all, the mother opted not to chase Sandra down? Or was it that she was told in retrospect? That makes sense.
Second of all---I don't even have a second of all-- yes I do:
If Sandra DID tell her little friend that she was running away, could this be something that Melissa goaded her into? Filling her head with either exciting plans and/or resentment toward her parents? Kids can be very gullible at that age, or at least I was. It'd be a way to get Sandra to go with her willingly if they indeed left the MHP, which I believe they did. I mean, why bring Sandra's sister into this?
Maybe this has all been said, but maybe Sandra did "run away" at the very manipulation of the person who wanted to do her harm. That's why she left and came back (if that happened) perhaps, because she wanted to tell someone she was going away.
I know LE said the backpack never existed, but wow, what a smoking gun that would be in this case.