Sitter was last seen on September 19, 2007. He left his home in British Columbia and traveled to Chuckanut Bay, south of Bellingham, Washington, to go kayaking for the day. He has never been heard from again. Sitter's yellow inflatable kayak was found on the shore of Lummi Island. A life jacket was attached to it, and Sitter's keys and wallet were in the life jacket's pocket. A bag full of food and personal items that Sitter had taken with him on the kayak, however, was missing. An extensive search turned up no sign of him.

Sitter was a professor of statistics and actuarial science at Simon Fraser University at the time of his disappearance. He got bachelor's and master's degrees from the University of British Columbia, and a Ph.D from the University of Waterloo. He is presumed to be lost at sea. Washington police are investigating his disappearance.