A trespasser is trespassed from a property by law enforcement at the request of the property owner. This is called a criminal trespass.
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In most places, a property owner must ask law enforcement to trespass the person off of the property before someone is considered legally or criminally trespassing. In most places a warning, either verbal or by sign or other means, must be given before a person can be criminally trespassed, but that is not automatic as the property owner may choose to not enforce it.
not true. I work in contract security training guards. have for years.
Trespassing generally only becomes illegal/criminal when the trespasser becomes aware that they are in fact trespassing. For examples, somebody wandering onto private property from public property, they could become aware of the fact. For example, if you have to jump a fence to get to where you were, or you passed a ‘no trespassing’ sign, or if someone is telling you you’re trespassing; or, for example, you’re there to vandalize stuff, or maybe shoplift.
The act of “trespassing” somebody is simply informing someone that they’re presently trespassing. You don’t have to be a cop to trespass someone; property owners have the right (and, generally, the obligation,) to control whose accessing their property and for what purposes. you can be asked to leave by a property owner or the agents thereof at any time, and that act of being asked is called “trespassing”.
As for when it can be enforced… that’s when the person is aware of their presence being unwelcome. Doesn’t matter if the property owner is there or not, exactly. no warnings have to be given, for example, if it’s reasonable that someone shouldn’t be there. for example, you intrude into a nuclear facility, we’re cuffing you up and handing you off, no warnings given. Similarly, if a group is throwing a kegger in a private parking lot, it’s generally unsafe to go out and warn the group.
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.