The insurance industry in CA is highly regulated. While I doubt it's state law they have to auto renew your policy, I wouldn't doubt the state of CA requires them to offer you a policy renewal as long as you meet guidelines outlined by the state. A company must give (or at least attempt to give) fair notice before cancelling a policy. Regardless, sending a policy renewal offer by mail is pretty standard for every company.
Renewals include a bill and usually state that payment is considered acceptance of the policy renewal and terms outlined in any addendum. If you were on an auto-pay service or allowed Geico to auto bill a credit card and never cancelled either, and never gave notice to Geico, you may be out of luck as well as money. If you didn't pay (or didn't inadvertently allowed them to get paid), you've got a better case.
Either way you can contact the California Dept of Insurance (
http://www.insurance.ca.gov/ ) to ask them about your situation and it's legality, or to file a complaint. You can also call Geico, ask (demand) to speak to management and mention filing a complaint with the DOI to see if that gets any results first.
The release of credits is highly recommended as it will not only save you from parking tickets, but could also save you from being named in a law suit if the new "owner" gets into an accident with a vehicle still under your name. We also take a release of liability as proof of sale, but I can't say if Geico will...