It is not criminal trespassing unless you remain on the property after being asked to leave by an employee, owner, etc. OR if you have been told by an employee, owner, etc. IN THE PRESENCE of an officer, that you are not allowed on the property, and you then remain or come back on the property. If there are signs around about trespassing, that is a different story.
I've learned that law enforcement officers will say whatever they think will intimidate you into doing what they want, whether it is true or not. Best to just apologize and leave because an officer can always find some reason to haul you in if he needs to, whether legitimate or not.