Here's the real (my) answer to any sign...legal or not if caught.....
"I'm terribly sorry, I did not see it, I will leave imediatly, BY"
Don't debate, argue or anything else.... if they want to discuss it with me they will have to while following me out the door.....
If they are calling the law then tell them you'll wait for them outside by your car since they don't want you inside and It would be trespassing for you to stay there. If the law is called I would not leave from the parking lot... in fact you might want to call the dispatcher (sheriff or city police) and let them know where you are and what you look like.....