Joan was last seen in King County, Washington on August 10, 1962. She planned to meet her sisters at the World's Fair in Seattle, Washington, but never arrived. She has never been heard from again. Her car was found abandoned several days after her disappearance, with no signs of foul play and no indication of Joan's whereabouts. An extensive search of the area turned up no sign of her and a private detective her parents hired was also unable to locate her. Joan was declared legally dead in 1975.

Joan was married to Robert Hansen in 1962 and the couple have two children who are now grown. The Hansen children grew up believing Joan had abandoned them. They now believe their father murdered their mother. The police have long considered him a person of interest in the case. Robert committed suicide in August 2009. He never faced charges in his wife's disappearance. In his will, Robert said his wife had disappeared years ago and, if she was alive, she was to inherit nothing. He also disinherited his children. In December 2009, his son filed a wrongful death suit against his estate. The suit is pending.

For years after her disappearance, there were rumors that Joan's body was buried beneath a barn on a farm in the Kent Valley along the Green River, which was once owned by the Hansen family. The barn had a new concrete floor poured around the time of her disappearance, replacing the old dirt floor. The barn has since been demolished and is now Kent-Des Moines Road. In 2006, at Joan's son's behest, investigators examined the road with ground-penetrating radar. No evidence was located, however. Joan's case remains unsolved.