BTW I've heard of guys reverse engineering metal detectors on a low budget, like under a few hundred dollars, to detect natural gas and oil.
The big oil guys can find deposits miles under the ground just by flying planes overhead, and have mapped the whole world out as well.
The tech is quite old now, and uses very little power to send signals down into the different layers of earth. You can buy commercial units for not all that much.
Well, marijuana and a lot of other drugs consists of natural oils. Same with humans. If I can build a underground natural gas/oil detector for a few hundred, I don't see why the gov couldn't detect every last person or drug going under a wall.
Oh wait, they probably already do
Here's some of the "latest" commercial tech, probably not cheap by any means. The life tracker one cand find life 40 ft under. Suppose they could have a mini train track with multiple GPR units zipping up and down the border.