If you click on the destroyer's icon on the nav map, you will see some rings drawn around it. The one with a triangle piece missing at the stern of the ship is her hydrophone coverage. If you are in the missing area astern of the ship, you are in her baffles. In other words, she can't hear you over the noise from her own engines. The smaller half circle at the bow of the ship is her Asdic, or active sonar, coverage. You don't want to be in that area.
Sven