Well perhaps a place sighted you and reported it? And your speed could determine how easy it is to see you.
Oh and which mods are you using? That greatly affects the LoS of the AI ships in game.
Me, usually if I sight a merchant before I know he's there (be it from hydrophone check or Bdu report) I determine his course and speed if I can, see if I can get ahead of him and if its worth doing it. Then I try and plot a nice intercept course to get me ahead of him.
Theres no point in submerging if he's ahead of you or else you'll just lose him but if you're ahead then its not a prob, specially during the day.
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