How did they find me?

Originally Posted by Schwartzritter

Having your navigator ping for depth marks you for destroyers. They can hear the ping, too. Along with what everyone else said, it sounds like everything simply went wrong for you. Also, Type II subs are poor boats for such a heavily patrolled area. Along with their sorry performance, you simply do not have enough torpedoes to be effective.

Normally yes, but from what everybody says here on the forums, the enemy actually doesn't get alerted to your pinging them or pinging for depth. Just another unfinished component to the game. You can thank UBISoft, I guess.