Sometimes it happens your watch crew cannot see things that you can, especially in foggy weather or at night. Same thing about the hydrophone operator, who can detect ships up about 20 km, while you yourself might hear a propeller sound from a distance of more than 25-30 km in favourable conditions.
If this really annoys you, I suggest downloading a mod for GWX which makes your crew a bit more watchful.
http://www.subsim.com/radioroom/down...do=file&id=959
By the way, ghost ships do exist in GWX, take a look at this thread.
http://www.subsim.com/radioroom/showthread.php?t=143344