Ok finally some "meat"on the subject.
You need to be low on fuel and or torps plus complete a certain number of missions in order to get a RTB....got it.
The reason I never got one before 1.5 gold Edition was due to lusy manual targeting in those versions. I ran out of torpedoes loooong before fuel ever became a problem. With 1.5 and 100% manual targeting success I have the confidence now to use only 1 or two fiash per kill and so request more missions per patrol...thus the "message." Now it is clear.
..btw..not at all mod related..these messages...