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? Where would it go from there? I'm guessing it wouldn't be as simple as, "Okay, just thought I'd ask, but you don't have to answer. Have a nice day."
Since this is probably not a very common incident, I highly doubt that security guards would have received any specific training on this. My guess is that you could probably get into the building without answering this question, if you were willing to escalate the matter to a supervisor a level or two higher and stand your ground and wait while they tried to consult their legal counsel and figure out how to handle the situation.
In other words, it would be exactly like refusing to answer questions at customs. They will give you a hard time, call in a few supervisors, try to wear you down by making you wait, but eventually let you through.