Sound can travel MUCH further and fast underwater, so in the event of an implosion, it might create enough of a spike in hydrophones that is statistically significant. Also the experts might be able to analyze the signal's signature to confirm (or bet) it's an implosion.
Crossmatch that with the time of loss of communication and it's safe to assume that it's it.
Indeed, and moreover, the ability to detect underwater sound is probably aided by the lack of localized turbulence (wind noise in microphones) and the degree of sophistication sought after by navies due to the detection and counter-detection of military submarines.
Without any actual knowledge, I imagine that a ship or sub could be festooned with hydrophones, enabling it to detect faint noises, but also to determine their direction from phase information.
Crossmatch that with the time of loss of communication and it's safe to assume that it's it.