Deep Cut found the map leading toward Splatoon Raiders’ island trip in a basement. That discovery provides the clue that sends Shiver, Frye, Big Man, and their mechanic toward the Spirhalite Islands.
The basement detail matters because it establishes the expedition’s origin before the group reaches the islands. Once they set out, their helicopter crashes, forcing them to survive and investigate the region rather than simply follow a straightforward treasure route.
The map was not found in Deep Cut’s broadcast studio, a museum, or a library. Deep Cut are best known as the hosts of Splatsville’s Anarchy Splatcast, so the studio is an especially plausible distractor, but it is unrelated to the map’s discovery in Raiders.