When I get the stutter, I dive and listen myself on the sonar, you can pick up a green light on the sonar then click to ID and receive info. You can hear a ship before your radar detects it. If you don't hear anything, it's most likely a sampan or fishing boat in the area which your radar won't pick up.
The info will tell you if it's closing or heading away, but it'll always say merchant even though it may be a warship.
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