California Code, Streets and Highways Code - SHC § 18129

If the owner can not be found, after due diligence, the notice shall be posted in a conspicuous place upon the land at least 30 days before the date stated therein, at which the application for a deed will be made. The person applying for a deed shall file with the tax collector an affidavit showing that notice of such application has been given, as required, and if the notice was not served on the owner of the land personally, that due diligence was used to find the owner. The tax collector shall receive and file such affidavit in his office.