The bar for trespass is met only if the perpetrator has been “forbidden” from accessing the property by the owner. This does not have to be in person, or verbal. A “keep out” or “no trespassing” sign would suffice, and this is why such things exist. In this case I would be immensely surprised if there weren’t some kind of employees only, authorized personnel only, or keep out sign posted on whatever method of ingress was used to reach the inside of the sign.
The intent of this is clear, it’s so nobody can get done for merely setting foot on a property in some situation where they didn’t realize they’d left public right of way or a property where they had authorization to be. You have to tell the person to GTFO (either preemptively or upon discovery) and if they don’t, then they can be arrested.