I worked in an executive branch building for many years with a sign directing visitors to the public assistance office and frequently found visitors wandering around other parts of the building with a lost expression of their face.  When asked about the sign, they said they did not see it.  I believed them and directed them to their destination.

An alternative to consider is to post a sign asking CHP holders to please refrain from carrying concealed handguns in the building.  A polite request is more likely to be complied with than the current prohibition.