I'm not sure about the 1st part of your post but the second concerning the dotted circle you can see on the map when you've zoomed down to one of the close up resolutions represents the ships horizonal view....Under perfect conditions the ship - not just destroyers but most all ships have horizonal borders - will be able to see out to that distance.
You may notice that if you cross the dotted line your stealth meter will start to lose it's bright green look indicating that you are "potentially" coming in range and becoming visible.....although 20km you say...hmm....i presume then that would be it's radar coverage...not sure if the human eye can see detail at 20 k's even with bino's....
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