Theriault was last seen in Irvine, California on December 2, 1998. He has never been heard from again. He left his money, credit cards and vehicle behind when he vanished. His live-in girlfriend and coworker, Judy Valot, was convicted of second-degree murder in his case in August 2000, nearly two years after Theriault disappeared. Theriault's remains and the murder weapon have never been located. Authorities presented several drops of blood found in Theriault's garage as the key pieces of physical evidence during Valot's trial. They argued that Valot killed Theriault because she believed he was having affairs with younger women.

Valot was sentenced to 15 years to life in prison. In 2002, her conviction was overturned. She was tried again but the jury could not reach a verdict in the case. In June 2005, Valot was tried a third time and convicted of second-degree murder again.

Investigators believe that Theriault's body is buried in the Mojave Desert between Irvine in Orange County, California and Blythe in Riverside County, California. Searches of the locations have not produced evidence as to Theriault's whereabouts. He was employed as a maintenance man at a Ford warehouse in 1998.