Legal Question in Criminal Law in California

The police response is likely correct, otherwise they wouldn't have told you that. They don't need to make up stories, or the law. That applies only to trying to get criminal charges filed. Nothing is stopping you from filing a civil lawsuit against the 'thief' and trying to PROVE your case to a judge and jury using whatever evidence you may have. If it wasn't registered, as is required by law, you will have to prove it was yours, and that the 'friend' took it improperly. Go for it. If you claim value at less than $10k, you can file in your local small claims court.