Kniffen was last seen at her residence on McClutchin Drive (now called Blackmon Drive) in Jay, Florida on September 14, 1985. She has never been heard from again. She lived with her two adopted children and her husband, a minister, at the time of her disappearance. She had had two biological children but they died of an illness before she vanished. She is described as a devoted mother.

There are conflicting accounts as to the events leading up to Kniffen's disappearance. Although some accounts state that she left her home voluntarily, possibly going to New York, foul play is suspected in Kniffen's case. Investigators are treating it as a homicide. Her case remains unsolved.