Sadly, I've played this game for years and I still don't really know the answer. My latest opinion is this - if they've gone after one of your decoys, and they're pinging it like hell, you get the message that they're pinging you. But don't run in that case - just listen to the hydrophone - if they're not on an attack course towards you, then don't make any noise - just hold tight. If you haven't blown any decoys, then he's pinging you.
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