[Jonty30]

I’d like to know how they found out he had a bunker? If I had one, nobody would know.

The door to it would be covered with a carpet and a desk (or something else) would be placed on top. That’s how it would stay unless I were down there doing something.

[Trod Upon]

Probably airborne ground penetrating radar. It's now cheap and small enough to be put on tiny drones to ID landmines. More powerful, aircraft-mounted units have been around for quite a while now, to say nothing of satellite GPR. If you have a bunker, I guarantee someone knows about it. With kooks like Napolitano on the loose, every house with a basement foundation is probably in a database somewhere by now. There are obvious implications for those who would consider burrying weapons or items of value in advance of tyranny.