Hidden islands are islands that are difficult to find. They are likely unmapped and in narrative worlds often are only found after a crew becomes lost at sea due to a storm. They may exist in the center of perpetual fogs, storms, whirlpools, or treacherous waters. Hidden islands may also be kept hidden by a person or group of peoples for reasons of their own. Hidden islands may contain ancient civilizations, long-thought extinct animals, behemoths & leviathans, or ruins from another age. They may be the sites of wonderful relics, temples, or powerful magics. They may host secret laboratories, military bases, super villain lairs, or bunkers. There may be skull rocks on them.
When designing hidden islands, first consider the island itself. Hidden islands may have all of the typical island elements frequently found. Also consider who or what keeps this island a secret and the means by which they go about doing so. If this island has been found before there may be clues (shipwrecks, survivors, skeletons).
If your world has satellite imagery, why doesn’t this island show up on any maps?