Security sensors are motion detectors. Some of them can be calibrated to detect human-sized motion, but that's about it. Human-sized motion includes trees and shrubs blowing in the breeze, blowing and drifting snow and dust, fog, heavy rain, clouds passing overhead and producing a moving shadow on the ground, legal human beings who happen to be near the border, game animals such as deer and bear, the list goes on and on.
I worked with such security systems for 8 years. Many times the newest gee whiz stuff, when tested in the real world, turned out to be "Oops. Didn't know that would happen."
Sensors work great in a relatively small, controlled environment. Over vast, uneven terrain like the border, not so much. You'd have to level it and remove all obstacles that play havoc with sensors. Something like a paved road with a fence on both medians. Over that distance, it would require a lot of manpower to respond to alarms, many of them false due to the fickle nature of sensors.
Agreed, there’s no way a system like that is going to work with hundreds and hundreds of miles of border.
With the cost over runs they’re having a good solid real fence would probably have been cheaper anyway.
Then they could just patrol it with A-10’s. LOL!