I had the same thing happen. Didn't know until I heard the explosion, and the damage reports came in...
Must have been the sonar man's nap time, since he never told me that my own fish was comming back around.
Outsid of that, it was one of the coolest experiences I've had so far with SH4... I sweat now when i fire a torpedo, then listen from the sonar station, in case he falls asleep again