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In: Other
For Hawaii, which is one of the more remote groups of islands, two possibilities come to mind:
Pumice. Hawaii is volcanic, and while most eruptions are the familiar rivers of lava [there are exceptions](https://en.wikipedia.org/wiki/Keanakakoi_eruption). The maps I see online show a current flowing westwards from Hawaii, so if the Polynesians encountered relatively fresh pumice coming from the east they would suspect there might be land there.
Those same currents would carry rafts of terrestrial vegetation torn loose by heavy rains, which would be an even stronger indication of land to the east.
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