DMH has a "no-loaded-weapons-inside" rule and I can only guess that someone was attempting to unload/clear before entering. Never mind that having a loaded mag or any ammo on your person is also verboten...
DMH has no such rule. My wife occasionally drags me to a jewelry show at DMH and there are no 30-06 signs posted except when a gun show is there.
This may not be exactly correct, but from what I gather, the promoter has to have insurance before the venue will allow him to use the facility and the promoter's insurance company is who imposes the "no-loaded-weapons" rule.