Try this:
PURE SPECULATION:
Gannon is or isn't sick. Stepmom is home, Dad is stationed out of state, leaving her as a virtual single mother, with three kids in the house, one hers, two not. Do or don't add some marital strife, caretaker resentment, etc...
Long night with a sick kid perhaps, no sleep... next day, can't we see a sick child refusing to go to school or wanting to, a sick child wanting to go play, maybe texting his dad or not, but ultimately being frustrated with step mom's noes or yesses, and blurtng the words probably every stepchild has blurted -- "you can't tell me what to do! You're not my REAL mom!" Or "you're not my dad!" Or "you're mean, I don't like you, I'm going to run away."
IMO to an insecure, self-centered stepparent, that could strike just about every reactive nerve there.
"Oh, yeah? I've TAKEN CARE OF YOU FOR TWO YEARS! That's how you want to thank me? Get in the car, I'LL SHOW YOU running away."
....
This case unnerves me. They are every family, living in every fresh development, in Any City, U.S.A. How does it go from hiking and happy and normal, to this?
Let today be the day LE brings Gannon home.
JMO