Osborn dropped off at the Devils Lake Trailhead by his father at 6:00 a.m. on June 17, 2002. Osborn is an experienced and athletic outdoorsman and planned to climb at least one and perhaps all of the Three Sisters mountains, a hiking distance of about forty miles. He was seen near the summit of the Second Sister, half an hour after his father dropped him off. Osborn was supposed to meet his father at Lava Camp Lake Trailhead at 9:00 p.m., but never arrived and has never been heard from again. His father reported him missing that same evening.

At the time of his disappearance Corwin was carrying a heavy-duty reflective space blanket and about one day's supply of food and water. An extensive search of the Three Sisters mountains turned up no sign of him. In the days following Osborn's disappearance, temperatures in the area dropped below freezing. His case remains unsolved. He is originally from Bellevue, Washington.