It seems you have sumbled across the Passenger liner route. It you stay in the area you will get further contacts. There is a glitch in when you sink a liner, a ghost may appear and despite sinking them, you will get no credit.
A little tip, if you hit a passanger liner with a single torp, or a couple of deck gun rounds it will slow to around 7 knots, making it a easer target. If you use the deck gun, aim at the water line, especialy the high side of the ship if it has a list. (the side more exposed)
