OK, so you don't really care about people passing between two points; all you need to know is when there is someone in this "wide empty place" - yes?

So we have the possibilities of PIR detectors, or Radar detectors to detect intruders in this "exclusion zone"

Depending on how paranoid your client is, you might also want to add beam-break detectors along the perimiter fence so that you can detect if someone is climbing over the fence before they actually get into "no-man's land"

You might want to make it an electric fence (if that's legal).

Note that you may also have to comply with Police and/or Insurance requirements...