I finally have my theory hammered out. Did it take me long enough? Lol.
Somer leaves school with her siblings, probably already upset about the fight that she had earlier. Her sibs bring up the fight, probably in terms of "You are going to be in so much trouble." She lashes out, swatting one of them with her backpack, and then runs. She looks for a place to hide, probably just for a little while and makes a detour into the bathroom. She knows that her sister might look for her, so she hides in the men's bathroom, thinking she won't look there. After her sibs go past, she starts to have the common fantasy that every kid has...what if I were big enough to live by myself? What if I didn't have to go home at all?
She walks over, talks to the guy working on the house, maybe intending to ask him when they'll be done, trying to act adult, so she tells them she thinks they are doing a good job. Probably loses her nerve on asking if a kid can move into a house, or he says something to her about running on home and it spoils her illusion of being an adult. So she leaves.
This is the point that it becomes theory only. Instead of heading home, she goes to see a friend. I believe that this friend is someone known at least in passing to the family, but probably closer than just passing. He is someone that she trusts and has probably run to in the past on her short disappearances. Whether this person is a family friend or one of the RSO's living nearby, I don't know. But I think she went to him. There was no snatch and grab, she simply made a short walk before her siblings came back out looking for her.
Either, she is so upset that he thinks Somer won't notice that there is anything different about the way he is treating her, or he just decides that today is the day to take it from grooming to actual molestation. At this point, he could have accidentally killed her during the act or he could have intentionally killed her afterward.
I don't think he would have dumped her in his own trash, even if he lives in an apartment and has a dumpster. So I would think that he lives in an area that is far enough away from Somer that he wasn't caught right in the heart of search territory, but close enough, on foot, that she could have gotten to the house within 5-10 minutes. Or he has a legitimate reason to leave at a certain time, like for work, and he dumped her in the trunk for a short ride.
Alternate theory being that she tried to hide out behind the burned house on Gano, until she could walk to her "friend", and they found some evidence of her there outside, which spawned the removal of all potential evidence inside.
Prove me wrong.