So you need two cement trucks.

You can also find them with ground penetrating radar since they tend to use sand as that insulating layer. From underneath they open a hatch and drain the sand out to clear the escape hatch. A big block of sand is plain and obvious to identify on a 250MHz system.

Every bunker will be built by people who won't be invited to live in it. The locations of all exits, vents, water supplies, etc. won't stay secret for very long.

That's where the bee truck comes in.

Can also go the brute force route and overload the structure a lot of the time. Park a couple water tankers, a crane, whatever on top. They're usually not made with the intention of supporting an extra 100tonnes