I have run across situations where heavily loaded ships faltered in storms and sank without having any damage inflicted by me. the sinking icon was grey.
additionally, some convoys are known to come under attack by one or more AI U-boats if you are using GWX - it is possible she was damaged in an earlier attack prior to your discovery of the convoy.
I have encountered a number of convoys with some of the ships burning, scar markings on their hulls, others listing badly but still keeping up.
so it is possible an AI U-boat is the culprit
sadly, they o-nly strike with the deck gun on the surface so running gun duels dont last long especially with armed convoys with escorts. they rarely sink a ship, but often do some damage to a few of the merchants before the escorts fend them off - and by that i mean destroy them.
I once ran across a Type VII Uboat heading at flank speed toward the french coast.
it was burning at the conning tower and about every 1 or 2 KM there would be a big explosion which would leave floating debris. the sub however kept chugging along as fast as it could... i followed it for a while but decided to return to course eventually instead of burning up my fuel. but that U-boat continued on as fast as it could with a big explosion and fireball every couple of minutes leaving behind it a bread crumb trail of floating crates spaced every couple of KM it was interesting
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