I have had this concern as well. The timing of NC's disappearance does not make sense if the couple was intending to disappear. If this was a preplanned collaboration, they'd want their departure to go unnoticed for as long as possible to give themselves a head start. Leaving when she did gave them the absolute least amount of time possible before their disappearance would be noticed and become a major concern (minutes, rather than the hours head start they'd get if she'd snuck away from home the night before), which makes me think that:
A) there was some other catalyst that NC was not expecting -- a message or Snapchat that would not only be dire and immediate emergency but would require her to keep her departure a secret from her parents
-or-
B ) NC was coerced into a situation she knew was bad and wanted people to be looking for her immediately.
ETA: I do wish we knew when AA picked up SAH on that day, because that's quite a drive from Pullman to Seattle, and that could also play into why NC left when she did.