The many 'mystery' islands located throughout Polynesia are so called because they were found to be empty at the time of contact by European explorers, but nonetheless showed traces of earlier occupation.
Many are small and were abandoned due to ecological disasters such as cyclones and droughts, but the 'mystery' islands still played an important part in the colonization of the Pacific.
They were discovered and occupied when colonists first entered their part of the Ocean, and many of them acted as intermediaries between the larger islands and more distant ones. They were never intended to be permanently occupied but were staging posts in the chain of travel which fanned eastwards across the Pacific Ocean.
