Next time you go by there post a pic of the sign. Maybe it allows some discretion. If it says 'no illegal handguns', for example, you having a permit can carry. If it says 'no concealed HG then you can open carry (until you get asked to leave).
I don't think, in the absence of a metal detector manned by LEOs I'd pay much attention to a sign. YMMV, depending on local and state statutes.