They are refering to NRS 202.3673 which states:
3. A permittee shall not carry a concealed firearm while he is on the premises of:
(b) A public building that has a metal detector at each public entrance or a sign posted at each public entrance indicating that no firearms are allowed in the building, unless the permittee is not prohibited from carrying a concealed firearm while he is on the premises of the public building pursuant to subsection 4.
There has been discussion if the Statute includes "open carry". I think one would have a tough time beating it. If someone did I think that the ultimate result would be for new legiislation that would clear it up, and in my opinion for what it's worth, we would loose ground. Sometimes it's best to drop it and wait for the right moment. We have to gain our rights back the same way we lost them, incrementally.