Kueper was last seen leaving Buddy's Bar in the 100 block of east Highway 82 near the Highway 90 junction in Whetstone, Arizona between 3:30 and 4:00 p.m. on January 26, 2007. He lived in a trailer park about one hundred yards behind the bar. He has never been heard from again. His yellow and brown 1985 Dodge half-ton pickup truck with Arizona license plates numbered 884OVN was found abandoned at a wash in a remote area near the ghost town of Fairbank, Arizona, between Tombstone and Whetstone, four days after his disappearance. Authorities stated they found no obvious signs of blood inside the vehicle, but a hammer wrapped in paper towels and tissue was located. There was no indication of Kueper's whereabouts. He normally stayed within a five-mile radius of his home, and drove most places due to his difficulty walking.

A witness reported seeing Kueper in the parking lot of Buddy's Bar on the day of his disappearance. She said Kueper was being helped into an older, full-size pickup truck by two men who also got into the truck. The vehicle fit the description of Kueper's own truck. The witness said Kueper appeared to be intoxicated or ill at the time. One of the men who was with him is described as heavyset, with brown hair, facial hair and a cowboy hat; the other was thinner with dark hair. Kueper's loved ones stated he never drank to excess and should not have needed to be helped into his vehicle.

Kueper did not have any known enemies at the time of his 2007 disappearance. The only relative he had living in the area was his granddaughter; his two sons are deceased and his two daughters live in Illinois. He went to Buddy's Bar frequently and was well-liked by the other bar patrons. Kueper was in excellent health at the time he went missing. His disappearance is being investigated as a homicide.