Not Navy, just did some technical diving when I was younger. Most likely the signature of the implosion was picked up by the old fixed hydrophone arrays that form the backbone of submarine surveillance. It would have been like a really loud thunderclap as about 20 cubic meters of interior volume (and 5 people) tried to instantaneously squeeze into 0.05 cubic meters (about the space 2/3rds of 1 human usually occupies)… odds are hydrophones were still picking it up hours later and thousands of miles away.

And yeah, they knew pretty quick that a sound consistent with an implosion was heard at a time consistent with the loss of communication… not entirely sure why there wasn’t more communication of that earlier, though. Probably some fairly rich constituents pushing on the political side for a showy (and costly) rescue attempt that was always going to end up being a salvage and recovery operation.

Now that the engineering choices have become clear it’s no wonder this happened… they really had “disrupted” their way into a time bomb.