No caves found after digging a large area

A friend and I dug down to diamond level and began a quarry. We both went different ways, and after about an hour we each had massive quarries dug out. However neither of us had found a cave. I heard cave noises in a corner so we dug in all directions, still no caves. I know I wasn't hearing the quarry because it was lit up really well.

Why haven't I found any caves?

If you are not playing on a customized world, then what you experienced is just the randomness of the world generator - it is not your personal fault or a bug ^^ It is by the way not unlikely to hear cave noises from caves above, maybe that is what you heard.