wrt "trespassing" due to signage . . . the following represents my non-expert opinion only based on discussion with both MA and CT LE wrt door-to-door solicitations at an apartment complex that was posted and discussions at my first gun club when we were going to post the property with no trespassing signage (some research with law enforcement or a lawyer, don't recall which, was done before posting).
In both CT and MA we were told that No Trespassing signs MUST be posted every 50' around the entire perimeter of the property to have legal standing for any prosecution. One sign missing or 51' away from the closest other sign and you can not prosecute!
I suspect that a complete legal analysis of the citation above would conclude the above to be true.
Now posting every 50' around a shopping mall would be insane, but requiring EVERY entrance to be posted would be a logical requirement. So the failure of posting at every entrance via the anchor stores (Sears, Macy's, etc.) should destroy the mall's ability to claim trespassing by posting. [Even in posted malls, I have NEVER seen a no guns sign at the entrance to any anchor store!]
As a Constable, I've served a few no trespassing notices and can tell you (also based on discussion with LE) the way this works. The person has to be verbally told to leave NOW. If they leave, the business or person then writes up a "No Trespass Notice" to be served on the individual and the PD where the business is located. Once I serve those notices, any trespass by the individual can result in arrest on sight. Prior to that, the police will do nothing if the person is told to leave and does so. Only refusal to leave upon verbal notice can be used as a reason to prosecute. IANAL but this is what I have been told via multiple legal channels.
For those that are convinced otherwise the only safe thing for you to do is leave your gun and all knives/OC at home before venturing out just to make sure that you don't violate what you are convinced is the law. For the rest of us, we'll continue to ignore the signs and go about our business unmolested.