The U-boats operated in tight groups called a "wolf-pack". Each U-boat in the pack knows the relative position and location of the other U-boats in the group (to avoid confusion and collision between each of them). If one U-boat was simply unable to answer radio messages (by radio equipment failure or by purpose), the other U-boats knows that it is still alive because they will see it from their sonar. If it was destroyed or sunk, its blip will suddenly disappear from their sonar. If it tries to escape from the group, they will see the blip moving farther away from them. Then they can report these situation to HQ.
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