There are lots of other ways it's being monitored. But as for the flying below the radar, there are these along the southern border and Gulf of Mexico: https:/

There are lots of other ways it's being monitored. But as for the flying below the radar, there are these along the southern border and Gulf of Mexico: https://en.m.wikipedia.org/wiki/Tethered_Aerostat_Radar_System https://en.m.wikipedia.org/wiki/Tethered_Aerostat_Radar_Syst...