How do they know?

[GoldenRivet]

if you were not in silent running to begin with they will hear the reloading efforts started immediately after the shot.

also with a good ear into the hydrophone receiver on an escort - just right - they can in real life hear the hiss of the compressed air forcing the torpedo out of the tube. and can find you based on hearing this.

those are the only two plausible reasons i can come up with that they go to the exact position you fired from...

however i will also point out that i have fired several shots on a convoy and the escorts fan out in search patterns but never even coming close to me... this has happened several times.