Given the following context:

A woman's dead body is shown in a cluster of lantana bushes. 
Leon, a police officer, and Jane, have sex in a motel room. They part ways, and Leon and his wife, Sonja, attend Latin dance classes that the recently separated Jane is also taking.
Leon does not enjoy the classes. He is seen savagely beating a drug dealer during a bust. He has emotional issues but refuses to confront or admit to them. Sonja sees a therapist, Valerie, who has just published a book on her own daughter's murder 18 months ago. She and her husband, John, are barely on speaking terms; he later refers to their marriage as held together by their grief. She feels threatened by another patient, Patrick Phelan, who is having an affair with a married man, which forces Valerie to confront her own issues in her marriage to John.
Hoping to see Leon again, Jane purposely bumps into him outside the police station, and they have sex again despite Leon's reservations. Nik is upset that she is seeing someone because he is friends with her estranged husband, Pete, who wants to return home. Jane pairs up with Sonja in the next salsa class, which angers Leon, who ends their arrangement, which upsets Jane. She invites Nik over for coffee at the behest of Paula, with whom she is friendly and offers him money as they are struggling. Paula now starts to dislike Jane.
Valerie is coming home late one night and drives off the road. She is stranded and makes several calls to John, who does not answer. Finally, she is seen approaching a car coming along the road but never makes it home. Leon is the investigating detective on the case and looks into her office and notes.  Surprised at seeing his wife's name and file, he takes an audio recording of their sessions.

answer the following question:

Whose disappearance is Leon investigating when he finds tapes of his wife's therapy sessions? 

Valerie