The shotgun was put in the trunk of a patrol car driven by two deputies at the Lost Hills sheriff's station in Calabasas early Saturday, according to a release from the department.
The only item missing: The beanbag shotgun stored inside a black canvas carrying case.
"It was inside the trunk," said Deputy Crystal Hernandez, a spokeswoman for the Sheriff's Department. "There's a lot of equipment stored inside there."
The California Highway Patrol and the Los Angeles Police Department, along with a sheriff's search and rescue team and a helicopter crew, assisted in the ultimately unsuccessful search.
Investigators are continuing to look into how the gun went missing, specifically whether the trunk latch was broken or if the trunk lid became loose during the 32-mile drive.
Investigators are asking anyone with information about the incident to contact the Lost Hills Sheriff's Station at (818) 878-1808.