Escort ships protecting merchants in a convoy are always actively looking for submarines. Just because you can hear (see) their sonar pings does not mean they are getting a return from your sub. The numbers in the signature panel will tell you if active sonar from a particular target is likely to be showing your position.
Active sonar is all about distance, layers and the shadow zone. Passive sonar, is a different beast and is listening for noise that can identified as a target. Usually the passive towed arrays are what find you first with subs. Ships are noisy and easy to find so they aren't so concerned with stealth but are more interested in finding you and preventing you from having the time to target them first.
At least that's the way it's supposed to be. :-)
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