Anyboy Notice This on the Bottom Right of the Map?

I was on a solo run and noticed on the world map in the bottom right hand corner a picture of a skull above some waves. I decided to check out the location. When I got there, the location was nothing but water. No island, nothing under the waves, nothing at all. Anybody know what it might mean? I thought it was the symbol for the skull that appears in the sky but after playing the game in different sessions, I looked at the map and it's in the same spot every time.