Thinking logically for a moment I canot believe that any Sub Captain would risk going to PD if he thought for one second that the Op-For was within the range they would have to be to be seen by a submerged periscope. No way in the world.
However...
It is standard peacetime doctrine when surfacing in known shipping lanes to raise the scope while you are still well under to check for "shapes and shadows". The surface duct can be very squirrley in re. picking up ships that are close aboard, so a visual check is almost always done during a normal surfacing evolution.
JCC
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