To hear it the sound wave only has to reach the sub, for the ship sending the ping to hear it the ping has to bounce off and go all the way back. Which is why you can hear sonar pings about twice as far as they can be used to detect with.
So you can hear it with out being detected, but the sound wave still has to come to you to hear it. Sound is also very directional, and can be bent in water too among other things.
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