convoy escort ability???

[Wuss]

lo all,before i ask the question let me first recount what happened.
i attacked a large convoy last night,i was in near perfect position about 2km ahead of the lead ship,submerged and creeping forward on slow ahead.
i didnt even raise my periscope till they where about 4km away from me and blissfully unaware i was there.
i picked 2 targets and gave both 2 torps each,the nearest escort at that time was about 7km away from me.
i went deep and moving away at flank speed,i waited till i got the torpedo impact message then went to silent running directly away from the convoy.
despite that,the nearest escort was on top of me in minutes with one from the other side of the convoy not long after.
surely there is no way they could have found me so quickly.
what sort of range does their detection equipment have to be able to do that????
sorry forgot to say this was at night,the sea wasnt rough but not flat calm either