Buttonville airport is a small regional airport with few flights in/out on any given day. Anyone can monitor the tower on VHF and get a clue as to what is coming in, and some smaller planes do have ADS-B transponders and show-up on flightradar, or a basic SDR rig. It is flat terrain and any low-flying aircraft is visible well before it passes overhead. Can’t think of any excuse as to how this might have been allowed to happen.