Killion was last seen at the Target Meadows Campground, two miles north Highway 204 in Umatilla, Oregon, on the evening of August 15, 2007. He had been camping there with his wife and others. He is believed to have been carrying a folding knife and a plastic container with a red lid at the time of his disappearance. He has never been heard from again. An extensive search of the area turned up no indications of his whereabouts.

Killion resided in College Place, Washington at the time of his disappearance. He was in good physical condition in 2007 and could have walked a considerable distance. His case remains unsolved.