When a sub dives, it has a chance to lose 1 strength. A strength 1 sub may dive and lose that final 1 strength, and disappear. If I remember correctly, the sub lost this way will not be counted as a kill and will not record in the unit loss board. This will affect all subs, and happened often in my past games.
A theoretical possibility I hadn't considered,
does suffer a bit from lack of suspects (based on a switch sides in the save, allied air & naval assets in the area amount to 1 decrepit CA, 2 Tac, 1 Strat, 1 Figher)
ORIGINAL: Hubert Cater
I've taken a look and unfortunately cannot see any obvious reasons why the sub would have disappeared from play. I've made a note of it for my end for now.
all I can really hope for with as little hard info as I had.
I'll be sure to keep a careful watch over my submarines in the future :D