4 ideas
The body vanishes, and Ravinel then begins receiving notes, reports of sightings and other traces of the living Mireille. The whole haunting was engineered by Lucienne, allied with Mireille, to break him down for their own gain.
A person recruited into a scheme against a third party can be the scheme's real target. The plan he believes he co-authors is scaffolding for a second plan in which his own predictable actions are the moving parts. His confidence that he knows the plot is exactly what keeps him from seeing the larger one.
Someone who has committed a secret crime cannot take ambiguous evidence to anyone else, whether police, friends or a doctor, without confessing. So he interprets every sign alone, and the conscience that fills the silence produces far more fear than the manipulators could manufacture themselves. The manipulators need only supply a few minimal cues, because the guilty mind does the rest of the work.
An absence, such as a body that should be there and is not, cannot be answered, denied or fought the way an accusation can. It forces the person to keep open several incompatible explanations at once: she survived, someone moved her, or I am losing my mind. Holding them all wears down both judgment and sanity.