Cal. Penal Code § 18405

(a) If a petition is filed under Section 18400, the law enforcement agency shall inform the owner or person who had lawful possession of the firearm or other deadly weapon, at that person’s last known address, by registered mail, return receipt requested, that the person has 30 days from the date of receipt of the notice to respond to the court clerk to confirm the person’s desire for a hearing, and that the failure to respond shall result in a default order forfeiting the confiscated firearm or other deadly weapon. (b) For purposes of this section, the person’s last known address shall be presumed to be the address provided to the law enforcement officer by that person at the time of the domestic violence incident. (c) In the event the person whose firearm or other deadly weapon was seized does not reside at the last address provided to the agency, the agency shall make a diligent, good faith effort to learn the whereabouts of the person and to comply with these notification requirements.