[Sailor Steve]

I'm not sure how it works in the game, but in real life a thermal layer is not a cloaking device. Have you ever done the trick where you put a straw into a glass of water and it looks like the straw is bent, sometimes so much that the straw reflected through the water doesn't even seem to touch the part that's above the water? That's how a thermal layer works. It doesn't mask the sub, it refracts the ping so it seems to be somewhere else. If the game is working right they should know you're there, just have a very hard time finding out exactly where.

If you were running at faster than silent speed they might very well have heard you, and started pinging.