FYI, in most states, there is a right of adverse possession, AKA squatter rigths. It varies state by state, but the normal deal is that if a person "openly and notoriously" uses a piece of property as though its their own for set period of time (usually 20 years but can vary), the person can gain a legal right to continue this use. Generally its up to the person who is doing the trepassing to establish this right in court. There also usually needs to be improvements and use, like a wall or a driveway but in some cases its as simple as mowing the lawn. The "clock" stops ticking if the actual property owner can prove that at some point that they objected or allowed the use. This may be as simple as a certified letter to the trespasser from the property owner stating that the trepass is occuring prior to the 20 year point. Do note that if the public owns the property, this does not apply, but if the public is using private property, it can apply. Rockefeller center in NY is privately owned and allegedly they shut it down to the pulbic a minimum of 1 day per year to make sure that they dont lose it.
I dont in any way suggest that someone do this deliberately, but it does help to equalize the negotiations if push comes to shove. In any case if you are either end on a potential issue like this, its worth having a lawyer review as case law in every state is different.
My neighbor to save money long ago when he was purchasing the lot next to me decided to blaze and paint the boundary between my place and his lot, he did this without talking to me and inadvertently painted a completely new line that adds a triangular shaped piece of land to my lot that is 250' deep by 40 feet at its widest. Its just rough woods with out anything worth cutting so I didnt bother correcting him. He apparently is gettin ready to sell so I will see what happens the next time around.
Whenever I have bought property, I usually drive a steel pin down deep and out of sight next to the official surveyors pins. Unfortunately visible pins have a tendency on occasion to go "walking". A surveyor uses a special metal detector that can detect iron and steel pins down very deep and its always interesting when a deep pin remains where the official pins used to be. To give you an idea on accuracy of those detectors, I have found steel masonary nails pounded in old pavement covered with 18 inches of dirt. I also used to follow old barbed wires fences that were mostly a rusty spot in the ground. If you tried to dig the wire up it you didnt find anything that ressembed wire.