U-boats were often moved from their allotted patrol areas by Control as reports of convoy sightings were evaluated.
If or when you play GWX2.1 it won't really matter because all patrol related renown is no longer included. The only renown awarded is for destruction of enemy units.
In RL, most U-boats that returned (especially in late war) failed to sink or even see anything.
Because of the Allies ability to accurately read German code, most convoys were diverted away from known U-boat locations.
