I attempted to research this on my own, but found absolutely no results. Is there anything legally restricting customers from entering areas marked "employees only" and/or areas that aren't marked but are clearly not intended to be accessed by the public (such as a stockroom, employee bathroom, etc.)? Is it any form of trespassing or something similar that an employer could use as basis for legal action or calling law enforcement in an extreme case (e.g., disgruntled customer walks into the employee only area and refuses to leave until xyz demands are met)?
Most jurisdictions define trespassing as being immediately trespassing if you are somewhere
- you have been previously warned away from (kicked out of the mall) or
- where there are signs posted denying permission to enter
- certain places where it's obvious, such as a private home
- certain places defined by law as not needing signs because signage would be impracticable and it should be obvious - military bases, power plants, railroads, national borders, yada yada
Otherwise they require a cycle of confronting you, and warning you off, possibly with explanation of which property you are being warned off from.
So it's trespassing immediately if you knowingly barge past an "Employees only" sign. In the case of a genuine inadvertent whoopsie, it's unlikely the case would go very far. But if the facts and circumstances make the case that your invasion was intentional, then it is not innocent and those facts and circumstances enter into it.
Without a demarcing sign along an entry path, I think the retailer owes you a warning, unless the facts of the installation make it obvious.
So for instance if you got into a tiff with the desk clerk and disagreed that he was out of stock of the latest XBox and barged into the stockroom looking for Xboxen, I would expect your name on a police report. As a practical thing, police are not punishing crusaders, and if no harm is done, they will simply want to assure you don't do it again. They hope a thin blue adjudication will suffice, as they are partial to the lowest friction method.
So there is no "one free trespass" in law, but there is in practice.
Some jurisdictions treat certain activities (on certain types of land) as not trespassing; e.g. hunting or snowmobiling. This forces the signs to enumerate each activity: "No Hunting Or Trespassing". There are also many narrow exceptions in the law. e.g. utility workers to maintain power lines or cut trees. Some of those are Federal and pre-empt any state laws.
...a disgruntled customer walks into the employee only area and refuses to leave until xyz demands are met.
That is called trespassing.
Trespass is defined by the act of knowingly entering another person’s property without permission. Trespass | Wex Legal Dictionary | Legal Information Institute
The boundary between areas legally accessible by the public and not is marked by the "Employees Only" signage. "Employees Only" in that context means "No Trespassing." The property owners, the police and the courts understand that.
(Trespassing) is held to infringe upon a property owner’s legal right to enjoy the benefits of ownership.
Of course, criminal charges and possible civil actions vary among jurisdictions, and can be superseded at the local, state and federal level.
Criminal charges, which range from violation to felony, may be brought against someone who interferes with another person’s legal property rights. Criminal trespasses, depending on the venue of jurisdiction and case circumstances, fall under different subsets of law.
None of this discussion about the signs amounts to anything, really.
All of these questions will vary by jurisdiction, but typically, you can stand anywhere in any privately-owned store and be asked to leave and refusing to do so would be considered trespassing. The existence or not of a sign is irrelevant to a trespassing case.
The sign exists to notify the public that the area is not intended for their use. The implication is that ignoring the sign may lead to your being asked to leave. However, the sign confers no benefit or burden on an area of space in a store under the law.
Further, to the extent the signs mean anything to anyone, it is more for the benefit of the store itself. If someone is injured by machinery or something else back there, the existence of the signs demarcating that space from the public space will help the store's ability to not be liable for the wandering customer's injuries.