Marx and his mother, Charlotte Moriarty, apparently went on a walk at approximately 9:00 a.m. on June 21, 1977 in their hometown of Haaula, Hawaii. They departed from their residence on Hanaimoa Street. Marx's stroller was located at approximately 1:00 p.m. that day at a bus stop at 54-295 Kamehameha Highway in Haaula. There was no sign of either Charlotte or her son and they have never been seen again.

Charlotte's boyfriend and Marx's father, Mark Barnes, discovered the stroller. Mark did not file missing persons' reports with authorities until three weeks after Charlotte and Marx were last seen. Mark claimed that Charlotte left their home without warning prior to this disappearance and he initially believed this incident was similar in nature. Mark then told investigators that Charlotte was under a psychiatrist's care at the time she and Marx vanished and Mark was concerned for his son's welfare. Charlotte's family disputes the majority of Mark's statements. He made several inaccurate reports to authorities at the time, such as claiming Charlotte was his wife when they were not married. Mark also listed his son's age as "three months," when Charlotte's sister maintains that Marx was five months old in June 1977.

Neighbors said that Mark and Charlotte seemed like a typical couple from appearances and were sometimes overheard arguing. Mark now resides in California with his wife. She told the Hawaiian media that Mark told her about Charlotte and Marx's disappearances and also said that he and Charlotte were not married. Mark has never been charged with any involvement in Charlotte or Marx's cases.

Charlotte's older daughter had planned to visit her mother's family in Hawaii during the summer of 1977. Her family maintains that Charlotte would never had disappeared with Marx without alerting someone to her plans. Charlotte's daughter and sister have investigated their cases since 1977, working with authorities and psychic detectives. A psychic claimed in 2001 that she believed Charlotte and Marx had been murdered shortly after their disappearances and were buried on the windward side of Oahu in Hawaii. There has not been any evidence to support this theory. Charlotte's sister has started the process of having Charlotte declared legally deceased, which was suggested by Hawaiian authorities. Charlotte and Marx's cases remain open and unsolved.