The apartment complex is under no obligation to verify registrations (and they don't have access to DMV databases). That's why you're supposed to put the sticker on. Without a valid decal, the vehicle is in violation and they can tow. I agree, it was a crappy thing for them to do, especially if they knew you were on vacation - but they aren't going to have legal liability for towing an improperly tagged vehicle.
If the towing company notified you in accordance with state law, and failed to receive a response, and waited whatever the prescribed time is under statute before selling the vehicle, you might be out of luck. Perhaps Carl or another Californian familiar with the statutes can tell you the exact time frames in question.

