It is specifically related to the mysterious island that the Baudelaire orphans are trying to find. The radar displays a series of dots, each representing a known island, but the question mark indicates a spot that is not on the radar's map. This implies that the island is either:
* Uncharted: It has never been mapped before.
* Hidden: It is concealed somehow, possibly by fog, currents, or other natural phenomena.
* Moving: The island might be a floating island or a large, moving object.
The question mark adds to the mystery and suspense of the book, as it leaves the reader wondering what lies at this unknown location and whether it holds the key to the Baudelaires' escape from Count Olaf's clutches.