Mittendorf and his live-in girlfriend, Kristina Branum, disappeared from Hardin County, Tennessee on July 27, 2006. They were living with Mittendorf's mother, Cheryl Flatt, in the 15000 block of Highway 64 in Savannah, Tennessee. The pair went out shopping on the day of their disappearance. When Flatt returned to the apartment, the clothes Branum and Mittendorf had bought on their shopping trip were inside on a bed, but they themselves were gone. They have never been heard from again.

The dark green 1994 Geo Prism Branum and Mittendorf were driving was found unlocked and abandoned five days later. It was parked near a pond in a rural area in Lawrence County in central Tennessee, 50 or 60 miles from their home. One of their cellular phones was inside, as well as other belongings. A witness reported seeing the couple leave the car there, get into a white car and drive away. There were many possible sightings of Branum and Mittendorf in Tennessee and as far away as Oklahoma, but none of these sightings were confirmed.

Flatt believes George Baugus may have had knowledge about her son and Branum's disappearances. Baugus, also known as "Eddie" or "Bad Egg," had a criminal record for drug- and theft-related charges. He was murdered in the spring of 2010 and his wife is awaiting trial for first-degree murder in his death. Flatt said she found a lockbox in her home that contained items belonging to Baugus. She stated Mittendorf and Branum may have robbed Baugus or burglarized his home prior to their disappearances.

Branum had dated Mittendorf for approximately six months before she moved in with him. They disappeared six months after that. It is uncharacteristic of them to leave without warning and foul play is suspected in their cases, which remain unsolved.