I found this screenshot in my 'ole screenshots folder. It's from when I was playing Vanilla SH3 (I just finished my second patrol with GWX two days ago). The rectangles show the directions on which I had the sound contacts. And notice that next to the pointer is the 'Sunk Ship' icon, in colour grey. It was a fishing boat.
I've got also some screenshots of a small merchant that was rocking heavily (and headbanging
LOL) in rough seas. It was in a convoy that I was following for about a day. It didn't sink! (even though it had a small fire onboard).
So I guess it's a pretty random thing.