Don't know about the specifics of that case, but I can imagine that for low-frequency pervasive sounds, they bounce everywhere and thus any such equipment would measure them as "coming from everywhere", because they literally are. There's a source, but it's masked completely by reverberation effects. At some point with enough diffusion there is very little signal in the noise, if that helps..
I still don't understand why they could not locate it.
Sounds falls off with the square of the distance. A single, localized measurement can have a hard time pinning a direction, but a series of sample over an area will give you a 3D curve of intensity with a maxima near the source...
