Private property has no legal weight on signage. Private property means the business is owned privately, as in not owned by the government.
All a sign on private property means is that that is the stated wishes of the owner. "No guns" has no more weight than "No red shoes." At all times, for any reason that does not violate a civil right, whether there is a sign or not, you must leave when asked by any representative of the business. To refuse or argue is trespassing.
If a PUBLIC building, as in owned by the government (like the state DMV offices) has a sign or metal detector at every public entrance, than concealed carry (only) is prohibited. Even if the sign says "no guns," by law it can only restrict concealed carry, and you may still openly carry. Yes, the DMV is trained on the issue.
Federal government buildings are, of course, completely under federal law, and any carry is pretty much not happening.
Las Vegas city and Clark and Nye County buildings are in violation of state law when they refuse to allow open carry.