The smart ones have holes we aren't aware of, tiny foot prints, well camouflaged and underground to boot. Nothing visible to naked eye on the surface.

But with one FLIR thermal detector...one man portable ground penetrating radar you can find them and foul their air intakes and potential water supply with ammonia and sulfur or muratic acid if the emergency isn't nuclear or biological then these systems aren't closed off yet most likely. Then you wait.