A lot. Unlicensed driver, vehicle unregistered more than 6 months, driver arrested, and a few other possibilities. Assuming there was lawful cause to tow the vehicle, a search pursuant to a pre-storage inventory is lawful.What in California would have been a "lawful cause" to tow the vehicle?
The OP should have been told the cause for the tow and may well have been provided a copy of the CHP-180 at the scene. The storage authority would be on that document. If not received at the scene, a copy would be mailed to the registered and legal owners.