I think... That if they dont find evidence of a person in the seat, a person doing damage to the instrument panel, etc, then I think (think) that perhaps the plane could have been put in auto-pilot and pointed at the mountains at a low alt. setting.
I believe its possible to set some AP's to work on a takeoff run (if the climb is minimised). Thus if one landed in the desert, got out, had everything set, then reached in and pushed the throttle in, the 'scenario' would be put into motion.
Just a theory...
And thats if they dont find body damage on the instrument panel.....