Yep, I saw this ghost ship last night.
I remember I sunk 2 enemy shipping groups (15 + ships? refit in between.) before I saw this thing.
Gunner won't open fire on it, so I did it myself. After I hit it couple of times and + 1 torpedo hit, it stopped and nothing will hit it ever again. I even drove my sub right through it.
The name of the ship is - "What is it". I guess it's a joke from the developers.