Cal. Civ. Proc. Code § 1988

If a witness is concealed in a building or vessel, so as to prevent the service of subpoena upon him, any Court or Judge, or any officer issuing the subpoena, may, upon proof by affidavit of the concealment, and of the materiality of the witness, make an order that the Sheriff of the county serve the subpoena; and the Sheriff must serve it accordingly, and for that purpose may break into the building or vessel where the witness is concealed.