I have tested it out before and I am not sure if the AI is reacting to the sounds or the visual.
At night, against a non-alert AI, motionless or dead slow, you can come up to 500-700 meters without being spotted. However, if you are moving at full or flank, they can spot out to 1,500-2,000 meters. Are they reacting to sound or motion? its hard to say.
If you have IRAI, you can test it out using map updates on and seeing how far the visual cone of the freighter extends.
In RL, it must have been a concern, since I have read about U-Boats using electric motors on the surface so they would not be detected.
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