Young couple goes on van tour of US, boy comes back with van - no girl

I think in that case, he would have alerted authorities to start a search. The only viable reason to leave there without her, is you don't want to be there when they find her.
Yeah, if it was an accident, from the first minute when he doesn't go to authorities and say, OMG, my girlfriend needs help, starts his guilt. I may not know what to do right away in the middle of nowhere, but I wouldn't just shrug my shoulders, get in the car and drive out of state.