Is it my understanding that a front plate was taken off leaving the back plate on, and the car just backed up to the trailhead, so it would have a back license plate, but not one in the front? Are Oklahoma residents required to have a front plate and a back plate?
If so, and only the front was taken off, but the back was left on, maybe they did not want it to look out of the ordinary? (Unless Oregon requires both front and back plates, too? So someone would notice the car doesn't have a front plate, thinking it was an Oregon car?)