Nyleen accompanied her family on a picnic outing in the in the Helena National Forest in the Elkhorn Mountains on June 25, 1983 near her hometown of Clancy, Montana. She was playing with some children who walked ahead of her for a moment at approximately 4:00 p.m. When her companions turned around, Nyleen had vanished. An extensive search of the area turned up no indication of her whereabouts. An unknown man wearing a jogging suit was seen nearby just before she disappeared. It is not known if he was connected to Nyleen's case.

Nyleen's family was questioned about her disappearance and her stepfather was considered a person of interest in her case. Her mother has since died; it was reported she was murdered in Mexico in the 1990s.

In 1986, a person in Wisconsin sent investigators a typewritten letter saying he had picked up "a girl named Kay." The letter included details about Nyleen's case that had not been released to the general public. The writer explained that he had a good investment income, worked from home, and homeschooled Kay. He also traveled frequently with her all over the United States and in Canada and Great Britain. He said he loved the child and did not want to give her up, although he knew her family missed her. The letter was postmarked Madison, Wisconsin. An anonymous caller who claimed to be the author of the letter called the Child Find Network, a missing children's organization in New York, about Nyleen's case several times. The calls were traced to various phone booths, including one at a pharmacy in Edgerton, Wisconsin. Investigators stated the information in the phone calls and letter indicated sexual abuse.

Nyleen has never been located. The person who wrote and telephoned authorities has not been identified, and it is unclear whether this individual actually had anything to do with her disappearance. It's possible Nyleen simply wandered away from the group and succumbed to the wilderness, but her disappearance is classified as a non-family abduction. Her case is unsolved.