I obviously don't know the precise situation, but I would have thought that, with a bit of ingenuity and thought, one could minimise, or even eliminate, that possibility. However, it was only a thought!The problem with that is, it would leave open an area where someone could sneak up the side and put a bag or something over the pir.Couldn't you simply point a PIR away from the boundary, so that it would only detect movement on your side of the boundary?
I suppose the other question is whether you really need to be able to detect people when they are remote from your property - it's presumably only when they approach the property (and its PIR) that they become of particular interest!
Kind Regards, John.