This case is of interest to me bc it's local - Saratoga is about 30 minutes by car from me.
Whenever someone vanishes along with their vehicle, a possibility is that they've left the roadway (usually by accident rather than with deliberate intent) and they and their car have become concealed from view, sometimes in water but more often by terrain: think of the Jolissa Fuentes case. Jolissa vanished almost a year ago to the day last summer, but her car was so far down a hillside it wasn't spotted until October.
That had been my leading theory here, but the discovery of Katie's vehicle would seem to change that. And yet her car was said to have been found "on Skyline Blvd". That's a state route - highway 35. It is almost impossible that her car (or any car) would have remained undiscovered for a month if it was just left parked on the shoulder. Perhaps the stated location is inaccurate; perhaps the vehicle was concealed in some way; or perhaps it was not there for the duration. It's hard to speculate effectively without having those possibilities resolved.
So another factor that we can consider is her cell phone being left behind. The location where she appeared on CCTV is but a short drive from her home. It's the kind of road that local teenagers with cars would know well; secluded but not too far away. But what teenager doesn't chose to bring their phone when driving to meet a friend? Perhaps she wasn't intending to meet someone. Still, rendezvous or not, I think most teenagers would turn around and drive back home to retrieve their phone as soon as they realized they'd left it behind. Yet Katie didn't do that. While it's possible that this was an oversight, it seems more likely it was deliberate. Then the question that follows is, why would someone not want their phone? We could postulate a secret elopement - maybe her parents didn't want her seeing a certain person. So perhaps she felt that taking her phone would allow her to be tracked. But perhaps she left it behind because she felt she would no longer need it. (Both things could be true; maybe she'd obtained a 'burner' phone.)
But it's the possibility that she didn't need her phone that worries me. With the KTVU article stating, "Crews used ropes to navigate the steep and rugged terrain," I've become concerned that suicide is their leading theory. Many of us have wondered about the description of Katie as a "shy, introverted child" and the attempt to "protect her privacy". How would those things be relevant? Could it be that her mental health was believed to be unstable enough that any perception by Katie of critical attention (even if sympathetic, e.g. that from investigators) might have been considered detrimental to her survival? Obviously homicidal violence is not excluded but to date there's been no suggestion of this... and in such a circumstance one would expect the police to put their investigatory focus upon her vehicle rather than nearby hillsides and canyons.
Well, that's a wall of text and a lot of speculation. I sure hope that I am wrong; I hope Katie has run off without realizing how much people cared for her, and without considering how much anguish her unexplained absence would cause. Those are much the same kinds of things I wished for Jolissa Fuentes and her family. It didn't work out that way for the Fuentes family, but I hope it will work out this time...