Check also rule 11.2.2: "Once a raid is detected, it stays detected, even if it moves out of the arc and range of RDF or moves away from land/coastline hexes in England." So I think your raids are still detected regardless.
Yes it's simply a map limitation. Any detection check could make it hidden though (the above does not trigger detection checks, I think removing the comma after the word 'detected' may make is read a bit clearer) Splitting the raid or a random event could also trigger one. It would be very gamey though if the Lutwaffe plotted a split there to try and take advantage. We've already ruled RDF is in those hexes.