I think I read it on one of the u-boat sites that destroyers patrolling along the british coast reguraly depth-charged ship wrecks sitting on the seafloor. This might suggest that 24/7 ping was enabeled.
About the type XXI sneaking undetected inside a convoy wikipedia says:
"A few hours after receiving the cease-fire order, U-2511 spotted a group of British warships. Korvettenkapitän Adalbert Schnee approached to within 500 meters of the British cruiser HMS Norfolk without being detected."
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