Riddle was last seen at his girlfriend's residence in the vicinity of the 23900 block of west Coleman Drive in Congress, Arizona on October 16, 2005. He left after an argument. He returned to the residence the following morning to collect some personal belongings, but has not been heard from since. Riddle's vehicle was located on October 19 on a dirt road in the desert near 5 Mile Tank in Congress. Several days later, a pair of his shoes were found near where the vehicle had been. The shoes were located in a place that had been searched previously.

Authorities do not suspect foul play in Riddle's disappearance, but he is missing under unclear circumstances. His case remains unsolved.