The tresspass statute is very clear - not only must the person be given notice to leave, the person giving notice MUST be the owner or agent; in the power of attorney case cited above, police provided such agency notice in writing; further, as the courts have held this offense to require a willful act, not only is a bone fide if mistaken belief in authority to stay a defense, but I think it faily implied that the person ordered to leave be given notice that the police are acting as agent; finally, if the onwer is the city or its juridical entity, then preemption would appear to apply voiding the alleged rule against gun carry being enforced by resorting to tresspass law.
