Ahhhh.....you may be onto something Otto.
I was chased into the midst of this convoy by 5 destroyers - very calm seas. Since this was a very close quaters battle (300m to 500m) from any given merchant (all armed - yes), I was most certainly easily noticed. I do recall an awful LOT of cannon fire trying to take my periscope off so I think you may very well be right - this is a case of friendly fire. Although I like to think my torpedo's (every one got a hit) did most of the damage
BTW : grey/black plots of sunken ships are those you hear to sink, as opposed to watching them sink, I believe. I get a lot of these when sinking ships in poor visibility conditions (e.g. heavy fog).
Thanks,
Dud