Huh. I recall encountering massive random cavern complexes back in 02Feb when I was going around mapping the underground. (Artifact with the “map” ability is your friend. Kinda.)
I wonder if there was a connection to the sewer or subway lines? Most such caverns I’d found were at least around such lines. Not a whole lot of help, but might be worth thinking about. (Perhaps a player-audible-only dripping sound from sewer lines/drafty sound from subway lines, conveniently ignored in the lines themselves?)