Just take into account that because you hear the pings it doesn't mean they have detected you with the asdic, just that they suspect that you're in the vicinity , either because they hear you in the hydrophones or they'd spoted your periscope and then they switch to their most acurate sensor to get a hard fix on you, the only real indicator that they've precisely located the sub is when they accelerate to make a DC run, if you go ahead flank and they're just trying to figure out where the hell are you then you're just telling them "hey I'm here"...
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