Pretty sure a fair number of people knew the US Navy and others had hydrophones in place, they've always been coy about it though.

For interest:

* it's one reason we know so much about ocean tempretures and tangentially have great data on climate change being real, and

* they had some cool R&D vessels:

  FLIP was originally built to support research into the fine-scale phase and amplitude fluctuations in undersea sound waves caused by thermal gradients and sloping ocean bottoms. This acoustic research was conducted as a portion of the Navy's SUBROC program.