[tophat9000]

How the heck did they know that it was sunk by a mine if they had never found it?

I would guess Japanese after action reports besides they say it was sunk by a Japanese minelayer (ship) not a Japanese mine...The minelayer might have deck guns and depth charges to do double duty and deal with sub's attacking it

[Mr. Silverback]

Japanese after-action reports were often quite useless, but when they were useful, they were very useful. For example, in Submarine Commander, Paul schratz describes how the Japanese believed they had sunk over 900 of our subs, but then goes on to describe how he was able to use their reports of "confirmed sinkings" to solve a number of unanswered questions related to his war patrols--whether a ship they'd attacked had been damaged ofr sunk, that sort of thing. IIRC, he found that as far as the Japanese were concerned, he'd been sunk several times over.

I recommend Submarine Commander highly, BTW. It doesn't read like a novel, but it is still a very interesting read by an authentic American hero.