The only people who know for sure aren't talking. You don't react to a sub the minute you discover it in peacetime. It gives away too much information about t

The only people who know for sure aren't talking. You don't react to a sub the minute you discover it in peacetime. It gives away too much information about the quality of your sensors. They may have discovered it far from the fleet, or may have missed it, or anywhere in between. Also, during wartime fleets will be using active sonar. It's going to be a lot harder for subs to hide.