Dupont de Ligonnès Killed Himself.
Investigators believed that after Dupont de Ligonnès left the hotel, he went to the nearby backcountry — full of cliffs and caves — and killed himself. Police, firefighters, and sniffer dogs were dispatched to scour the nearby wilderness areas but to no avail. Over the years, several bodies have been found, but none of them were Dupont de Ligonnès.
In many cases of familicide, the perpetrator commits suicide right at the scene, so it struck investigators as odd that Dupont de Ligonnès left the scene of the crime. Perhaps he went around southeast France to say goodbye to spots that held good memories, but we see in the episode that those closely involved with the case — particularly a journalist (Anne-Sophie Martin) and a friend (Bruno de Stabenrath) — question this theory.