Since November, family and authorities tried to find Lauren Jessie Moss.
She had gone missing, and officials in Monterey County said they could not find her — until this week.
Investigators said they found Moss inside a car that had been parked outside a Wal-Mart store in Salinas for months.
Police checked video footage and saw the car had been parked there since December, KSBW-TV reported.
Moss was last seen in Seaside on Nov. 13, the same day she left a rehab center.
Salinas police officers said her death was likely a suicide.
Police say Moss' car had dark tinted windows and a sunshade over the dashboard, making it hard to spot anyone inside.
A Wal-Mart spokesman said the store does not have personnel patrolling the parking lot but does have 24-hour surveillance cameras.