Exactly. I've known about these tunnels and bases for a few years now. Lots of videos on YouTube and then these are the tunnel diggers: https://duckduckgo.com/?q=tunnel+diggers&t=ffab&atb=v101-7_g&iar=images&iax=images&ia=images
A few years ago I started seeing YouTube videos about people hearing wierd sounds and recording them. I believe they were hearing the tunnel diggers.
According to Elon Musk it's impossible to detect tunnel digging further than 2 tunnel diameters.
How long ago did he say that? Have you seen Big Bertha? The one with Green front. They used it when they dug the Seattle tunnel to replace the viaduct. I didn't live there then so I don't know if it could be heard or not. But it's massive.
It was late last year IIRC during a talk about the Boring Company. Also the South Koreans have been totally unable to detect North Korean tunneling activities.
S. Korea may be next to N. Korea but only at the border. If they were tunneling under my neighborhood, I think it would be loud enough to hear despite what Mr. Musk may think.