IANAL but from what I can tell NRS 218.542 is the only law against COMPLETE firearm possession in a state building, and that prohibition is only the state legislature or an alternate location where the legislature is conducting business.
NRS 202.3763 applies to concealed carry stating concealed weapons aren't permitted in a public building with a metal detector, but is silent on NON-concealed weapons.
I wonder what would happen if they asked you to disarm or leave, and your response wasn't one of disobedience but rather...
"I do not consent, but I will disarm if you say I have to under the law. I don't believe I am legally required to disarm, nor do I believe you're authorized to require me to do so. Would you please give me your business card and the name of your CO so that I may follow up with a complaint? Actually, do you have a complaint form I may take with me?Ahh, day dreams.
Also ,since you have no metal detectors I don't feel safe knowing someone may have snuck a firearm in without you knowing. Would you mind waiting with me or checking each citizen here to make sure no one else has a weapon?"