BBM
That is one of the main points, imo. Dylan would know that RN was not going to spend the entire day sitting at his grandma's, waiting on him. He knew that at some point RN was going to split and go meet up w/friends somewhere else.
So it makes no sense to me that D would just pack up and leave his Dad's, without first contacting RN to say he was on his way and find out where to meet them.
There are several reasons that he would contact RN before leaving his Dad's house, imo. Saying sorry for being late, checking where RN was going to be, finding out if anyone was in the area and might be able to pick him up, and tell them he was on the way.
As you said, the kids might have made plans to go somewhere, and why would D try to hitch 18 miles without finding exactly where to get dropped off? When you 'hitch' a ride, you don't often get door to door drop off service-- Unlike what he would get if he waited for his Dad.
I just cannot see him waking up and taking the time to watch cartoons and eat cereal, but not bothering to make contact with anyone before heading to Bayfield.
If he was wanting to go to Bayfield, and not waiting for his Dad, I believe he would have reached out to his friend in Vallecito first. Wouldn't there be a chance he was heading to town and could give D a ride?
I would see hitching as a last resort, not something he'd do before checking out easier options. JMO