Umm, not to belabor the obvious...
But it ain't just cops keeping hidden weapons banned by the California DOJ.
This long-time California resident left there in 1999 just as SB-23 went into effect. I took my collection of banned firearms with me, and all the illegal magazines they didn't want me to have. As an example, I had 300 each 30-round AK mags in my household goods shipment, alone.
One thing I heard from my fellow gun owners as I left California was that they simply were going to ignore the requirements to register their now-banned firearms. As I read the DOJ registration paperwork, which basically forced you to deed the firearm to them and lease it back until the day you died, at which point it couldn't go to your children or heirs, then I knew that civil disobedience would be the norm vs. the exception. I've been back a couple times since then, to visit family and friends. I'm not about to rat them out, but they have indeed ignored Lockyer since 1 Jan 2000.
Somewhere out there one can find a news article or three listing the DOJ's dismay at the abysmally few numbers of banned firearms that were actually registered with them, compared to the estimate of guns that were supposed to be submitted. It would appear that the DOJ was simply ignored by the majority of folks out there who were supposed to comply.
It was ironic to me that a short time after I left, I received a forwarded postcard from then AG Lockyer's office, notifying me of my requirement to register my AK/AR/FAL etc. within so many days, or else. I'm assuming they got the address through the 4473s of my purchases, which is onerous in and of itself. I sent the postcard back, telling them in no uncertain terms that they were more than welcome to try to take them from my home in Florida.
It's way larger than just cops that went underground. Compliance with Roberti-Roos was estimated to be maybe all of 15%. I'd wager compliance with SB-23 would be near that, too. Lockyer went on record to report they had all of 10,000 guns registered with the $20 fee before the 1 Jan 2000 deadline. That speaks volumes about California gunowners, and what they really thought about SB-23.