[WernherVonTrapp]

There's only two reasons that I can think of (off the cuff) why you'd get a triangle (in the fog) which subsequently disappears. If the ship is broadside for a moment and the triangle appears, then the ship turns, blading itself and creating a smaller profile, then the triangle disappears. This can occur also during low light conditions without fog.

The second would be commensurate with the ship passing briefly into, and then out of, visual range while you're at a 90 degree angle to the target. The reason I point to (any) bearing and lock is because, if your sonar man calls out (or is continuously calling out) the target's position, your torp will travel the last known sonar path if you don't lock onto a bearing.