there is a technique in the game where an escort will go doggo and just sit in the vicinity of the action. totally still. perhaps the designers are taking advantage of the fact that your hydrophones cannot detect a motionless surface vessel.
this usually happens when you attack a convoy/TF and there is more than one escort.
after you attack, the escorts will try and locate you directly and kill you. if you evade that counterattack, all but one escort will round up the remaining escorted ships and get them back on course. one escort will remain behind, totally inert, waiting for you to surface or at least show yourself.
the IJN escorts do it in the Pacific and when you go and play the Dark Waters megamod, the Brits will do the same thing.
you have to be careful after an attack.
a best practice is to make your attack, firing as many of your loaded torpedoes as you can bring to bear and then scuttle off at a 90 degree course to reload. at least 6 or 7 NM from the attack area before surfacing. beware of trailing escorts!
this practice also has the advantage of allowing you to exit the immediate vicinity and any waiting, silent, motionless escorts.
then, if you wish to attack the remnants of the enemy formation, you speed ahead to get in front of it, and attack again.
