There is a trick in the SC code that the sign must be absolutely compliant with the standard specified in the statute. Such a sign is almost impossible to create, certainly not by any amateur. Furthermore, there is no standard, language-neutral symbol for prohibition of firearms. The one you see at a post office is not per any standard. Some sign maker made it up. Lawmakers apparently assumed there as such a symbol, because it is referenced but not pictured in the sign specification section.
Don't be too concerned about signs in SC, but a sublessee's office door inside a larger building is his to control, barring any lease condition to the contrary.