The hidden map zoomed in. It reveals that the dotted line leads to a black dot, which has no island on it. However, this dot may be Rockhopper Island, but he did not draw it, or Penguins will find out the island. [citation needed]
However, if they are the three little islands, then the maps are incorrect. They're incorrect because the islands are located Southwest of the island, not Northwest.
Sources and References
↑Rockhopper's maps are always a bit wonky, so I am not sure how accurate they are. However, he is quite the expert when it comes to sailing around the seas and exploring new places. Therefore, I think it's likely that you are correct about the islands - Swashbuckler's Trading Post, Dinosaur Island and Shipwreck Island. -Club Penguin Support