I ran into 2 sub chasers which accompanied two cargo boats and when I blew one of the cargo's out of the water (lucky shot, pressed 'enter' just too soon but it still hit) they didn't even came looking for me.
Made note to self: "Sub chasers: don't" which now comes right after "Friendly fire: isn't" on my personal war fare guideline.
Then I targeted the one trailing the 'convoy' and thought I just missed that one. I never suspected the torp to go under it. Lessen learned here today! Thanks.