I vaguely recall that the Germans used tunnel-detection tech back in WW2 to guard their stalags. Even back in the Dark Ages a noble under seige sometimes asked one of his men to put a shield to the ground and listen for sounds.
I suspect that modern tech can catch the sounds / etc. very effectively at low cost IF we have the will to use it. Might even be how this one was detected just after it reached the border.
There are some fascinating books on WWI about tunnel warfare. Some hair raising stories. Britain had made a couple of large mechanical boring tank like vehicle that they were testing. One broke down underground in France and they just left it.
For governments, life was cheap back then. I wonder if it still is, but they have learned how to hide it.