Ok, with Stock game. Snorkel up + ANY scope up and your at high TC (anything over 32), and an enemy unit comes into your normal visual range (a bit less than what you would have at surfaced state with watch crew), and the game will drop you back to a Lower TC rate. This is the only warning you will get. The crew do NOT give you a message/warning. If your in the Nav map view, you should see both Ships and /or Aircraft that caused the game to lower the TC value.

Yeh, i have it set at that.but thats only when you have sighted
the airplane surely? Which leads us back to the whole argument again. I think ive gotten some pretty definite answers though, so im satisfied.

Thanks everyone for the replies.

I've been attacked at x2048. It's rare, though. It also means they're dropping bombs before you can react. Luckily I was using the Airpower Mod, and they weren't accurate.