I am not certain but either there is a bug in stealth or there is a trick I am missing.
The issue is with one certain player they are never visible to me until my sub sinks. This happened half a dozen times last night. They would be almost within the sea wolf speed ring and I couldn't see them. This is with both of us not using sonar and with them using ships or subs. They can see me though as they are firing on me when we are 1 to 2 inches apart. Their torpedoes don't show red triangles either until the very last moment. He says its working that way for him against me also but he obviously sees me every time before I see them as they are firing on me first.
One particular battle I attacked a destroyer close range at a dock, sinking it, I immediately attacked a sub right next to that ship, sinking it. I turned on sonar, saw nothing and surfaced. As soon as I surfaced, his ship turned visible, I was right at his bow but never saw them yet I was so close I could see docks without sonar, just not his ship.
On another occasion, I had attacked a ship but as I was out of ammo so I decided to move under the ice around an large ice island and circle around to allow ammo to restore. This trip took about 45 seconds to a minute. The entire time I was doing this I kept seeing torpedoes appear without the red triangle every 10 seconds or so. I thought at first it was from the sea side being fired by others in a battle and it was just coincidence they were tracking on me. After about the fourth decoy, realized it was this same person chasing me the whole way. I only saw who it was after they were almost on top of me, which was weird as we both had the same sub yet they caught up to me and they could see me the whole time obviously as they were firing away.
This same pattern repeated itself a few other times where I would never see their sub until after I was sunk. Even the torpedoes would stay masked until right before impact.
You cannot defend against what you don't see so I figure there must be some trick to doing this type of strong stealth where they can fire away and I never even see them. I questioned them on this and they said they were using no sonar also and the sub itself was stealthy. Whats weird is I could see them perfectly fine half a screen away with a golf submarine. This makes no sense that a golf sub could see them but my sea wolfs vs sea wolfs couldn't.
I can tell that others can see me when attacking without sonar a little over two inches away most ships as they start evasive maneuvers before I fire torpedoes.