I would normally do that, I have a religious crew and often return to port with a holy boat. But I was just south of Davao where air patrols are common, and get worse once you start shelling a ship. If I trailed it towards Palau for a night attack I would have left my patrol zone. Then I'd have to return to start the 72hr clock again to complete the mission. It was already close to 2am real time and I wanted to get all the way back to Pearl before I went to bed......so I let it go.
I had a good patrol anyways, I caught the southern Midway invasion force at the beginning of the patrol and got a CL and two 7k freighters out of it. Followed by many singles including two 10k tankers, hence no fish left.
That has happened to me so many times, that I started to think it was hardcoded into the games as a teaser. I used to come across unescorted heavy cruisers, aircraft carriers and large tankers, but only after expending all of my torps. Since it happened with such frequency, I started the practice of saving 3 torps for my return trip to home port. I would never use them against any small or medium target, only for the fat, juicy, ones.