The meanings of Exit and NO Exit or Entry and NO Entry are naturally opposite.
When I see an [Entry] sign, it means 'enter here'.
When I see a [No Entry] sign, it means 'do not enter'.
In this respect, considering the definition of trespassing, what would a [No trespassing] sign mean? Could it mean *free to enter? (No - No entry without permission?)
No Entry means the contrary of Entry. No Trespassing shall therefore means the contrary of Trespassing? How could appearance defy logic?
Are we putting the car before the horse? In order to trespass, someone first needs to enter without permission. If there already were a no trespassing sign before someone had even been entering, could the absence of trespassing be interpreted as a green light?