Rosemary E. Bachelor has a post up called "California's Confidential Marriages: Secret Unions a Genealogist’s Nightmare."
Confidential marriage means very secret. For nearly a century, the State of California did not keep records of these marriages. It is impossible to determine if they took place, unless, perhaps, the couple later—for personal reasons—filled out an affidavit and swore to it under oath. County recorders keep a list of such marriages, but they are not permitted to divulge them. Technically, a clergyman is not supposed to reveal them either. In a few very rare cases, such information has been found among private records of deceased clergy.