Hi Von Due.
I remember somebody mentioning that game generates for task forces waypoints that end up at open sea, and that means they vanish when reaching last waypoint.
I myself once chased two auxilary cruisers for a very long time and suddenly they just went to "Bermuda triangle".
I was somewhat frustrated.
What comes to your missing contacts, that may be just a bad luck?
I use similar 20km interval hydrophone checks myself and can recall not havin contacts for weeks.