I finished reading the thread but didn't click on all the attachments so I missed how the truck picture was discovered. I'm assuming on a camera, but which one?
That Subway table was isolated from anyone inside from seeing her sitting there, but she would have been very visible to anyone driving by, especially if she was sitting there by herself.
Even though she may not have a facebook page, there are plenty of children game sites where the kids can talk. Most schools now have school lesson sites that kids can get on to practice. I hope the LE checked the home computer to see if she was being courted by a predator. Also, check the phone records.
If her mom married the stepdad two years ago and they had moved from somewhere else (?), I wonder how long she has been attending this school.