There is some evidence the GRUNION may have gone down afer a surface attack on a Japanese troop transport. Allegedly, a 3-inch shell from the transport struck the GRUNION’s conning tower. The wreckage should be able to confirm or contradict this theory. I, like many, always assumed she went down fighting.
I would guess that a floatplane got her, since she was operating so close to Kiska where the Japanese had seaplanes based. Aircraft probably nailed more subs than any other weapons platform. I guess that the searchers have already discounted this. Perhaps the Japanese have no records of an aircraft engaging a sub. ‘Course a lot of aircraft went up over the Aleutians never to return & aerial radio sets weren’t very reliable either. I wonder if the IJN mysteriously lost any floatplanes during that time frame?