Fun fact you should know: If you have a "Proceed to grid co-ordinate blah-blah and sink shipping at your discretion - 5000 tons would be a nice start" mission, you don't have to be anywhere near that grid coordinate to meet your quota.
My coordinate location was off the tip of Madagascar. After spending ten days patrolling and only getting a pair of fishing boats, I headed for the Gulf of Aden. Found and bagged a 4-ship unescorted convoy and lo and behold, got the mission complete check-mark - 2000 km from my intended patrol location.
Not sure if that constitutes a bug and I'm pretty sure I don't want it fixed.
