…One had law enforcement only trespassing, one had owner/manager only trespassing (as in, a random employee can’t do it, only the owner of the property or the person who is on the lease for the land/building), and the last was so loose a patron of a business telling someone to get lost was almost enough for the person trespassing to be arrested.

honestly, in my experience, this sounds more like policies of the property owners (or employer) than regulations or laws.

Especially since while it’s not specifically codified in federal law, SCOTUS has routinely accepted that the right to exclude is one of the core property rights inherent in property ownership- and consistently ruled in favor of property owners exercising control over their property. (Indeed, there have been instances where, when it was deemed that a property owner should have taken action to stop a trespasser, and that trespasser then harms some one; that they had a duty of care and were negligent. For example, any of the times that a convicted pedophile got hired by a daycare facility,)

there are exceptions that are frequently codified in state law, for example, a hunter retrieving wounded game, or kids retrieving a Frisbee. or a dog owner cleaning up after their dog (and, indeed, the dog taking the shit in the first place,).

Further more, all “agent of whatever” really means is that they are someone who is duly authorized to act on another’s behalf. For example, if you have power of attorney over a grandparent, you are their agent, acting on their behalf; or someone employed by the federal government, to do… stuff… they’re agents of the government. (Fun fact, this is why FBI and other federal law enforcement officers are ‘Special Agents’.). Security guards are agents of the property owners, and almost universally allowed to trespass individuals.