Let's assume we have a certain wooded block that's two miles to a side in the country, and several landowners that own non-rectangular sections. None of it is marked "No Trespassing", in accordance with your assertion. It is well known that all of the landowners except one allow hunting on their parcel, no written permission required.
Now let's someone is walking on property owned by a landowner that allows hunting, and he crosses 10 feet over the unmarked boundary onto the property of the landowner that does not allow trespassing. He then turns around and retraces his steps back 20 feet, all without knowing he even crossed that boundary.
If the non-permissive landowner had a camera there, and took the video of trespass to the local prosecutor's office, do you think he could get the trespasser arrested? Do you think the trespasser could possibly be convicted if it did go to court?