[Jimbuna]

Allow me to hypothesise further.....If you got the credit you definitely caused it's sinking, that is a given, but because it was out of visual detection range the icon appeared grey.

Often you will see a vessel succumb to a mine, bad weather or air attack even (the game engine treats flooding in the same way as battle damage).

It is not too uncommon for you to torpedo a vessel but the final straw for it's sinking is the damage incurred by the mine or bad weather.

The usual exception being when you fatally damage a ship and travel out of rendering range before it sinks....you return later to the vicinity and it has been rendered in game again but the sunken icon represents the initial sinking.