I am not sure that was the point was it? By laying mines a submarine is creating another war hazard to navigation. So say 20 mines are sown in an area. Two weeks later a Norwegian freighter hits one. This is when the story starts.

To the Germans they have sown a small minefield in position X. So be it. To the owners of the freighter/allied authorities it is what happened next. If the ship got off an sos or did the crew/some of the crew survive. It would be a process that unravels slowly and retrospectively.