It's Edwin Starr. The Edwin Starr mod makes your sub magnetic and all the destroyers are attracted to it!
OK, here's what really happened. First of all you were in a direct path with the convoy. If you are in their direct path and you are not pointed directly at them, as soon as they get in active sonar range they will ping. So they did. When they did, they detected you.
What? Did I mean to say that they knew to ping you and then learned you were there? Must be a misprint or something. But no! Read
Ducimus' "SH3 & SH4 "Uber" AI Explained". There you will learn the secrets you need.
