Thanks to everyone for the replies. All very interesting!
Out of curiosity--and let's say it was hypothetically under California law--how would that go, in practice? Let's say that rather than answering "no" and showing the pocket pressed flat, I just said, in a calm voice, "I decline to answer."
How would a typical security guard be trained to respond?
These are not "security guards." They are law enforcement officers, i.e. sheriffs or police.
Where would it go from there?
They would probably construe your refusal as reasonable suspicion, enough to justify a Terry frisk. If you continued to refuse, you'd be arrested.
For what it's worth, the funny thing is that I'd left my metal-frame glasses in my shirt pocket the whole time, because I completely forgot they were there, and neither I nor the guard noticed them. Plus my wife realized afterwards that she had an 8" metal nailfile in the bottom of her purse that would have looked just like a dagger on X-ray and might have been considered dangerous on search, which they didn't notice, but they caught her bottle of Coke! (She thought no food and drink just meant no eating or drinking, not a ban on sealed food inside a purse, so she wasn't deliberately trying to sneak it in.)
This isn't the airport, it's a courthouse. These are professional law enforcement officers who (1) have discretion, and (2) know their job, which is to prevent weapons from entering. There is no war on liquids and I'm sure they can distinguish between a nail file and a dagger.
Do not make the mistake of confusing this search, conducted by real law enforcement officers, with the dog-and-pony show put on by TSA.