Ok, this explains something that happened last night. My REAL last night, not last night in the sim.
I was about 50 nautical miles east of Japan. I didn't see anything because I was submerged over 200 feet down and it was night. I got a sonar hit and it showed up on my map. Using the 3:14 minute rule, I plotted it's course and speed. It was doing a whopping 40+ knots. Even with no time compression at all, I could easily the sonar contact moving straight in my general direction. It was on a heading that allowed to cut the corner and head it off and so, I went for it climbing to periscope depth to take a look. Just as I reached periscope depth, the sonar contact just faded away and there was nothing in the scope. I thought that this was really odd but forgot about it. Now that i see this thread, I'm thinking darn darn darn. Serves me right for not running on the surface at night anyway.