(CNN) — On a bad day, you might lose your wallet or keys, but it's a lot harder to lose an entire islet.

However the Japan Coast Guard is planning a search mission after an uninhabited islet called Esanbehanakitakojima seemingly disappeared off the northern coast of the country.

The small outcrop was previously found some 500 meters off a village called Sarufutsu on the northern tip of Hokkaido island, Japanese daily Asahi Shimbun reports.

Its absence was noticed by author Hiroshi Shimizu, who produced a picture book on hidden islands and had traveled to the area in search of inspiration.

Puzzled, Shimizu asked local fishermen what had happened. An elderly local confirmed that there used to be an island in the vicinity, according to Asahi Shimbun, but it can no longer be seen from land or from nearby boats.

A Japan Coast Guard vessel patrols the Northern Territories east of the 'disappeared' islet. Issei Kato/REUTERS

However, other fishermen said that Esanbehanakitakojima shows up as an islet on navigation systems.

Authorities are concerned about its whereabouts as the islet is used to mark Japanese territorial waters in the Northern Territories, an area that is also claimed by Russia. Known to Russians as the Kurils, the islands were taken over by Soviet forces shortly after the end of the World War II.

In 2014, the Japanese government officially named 158 uninhabited islands to demarcate its waters.

Beneath the waves?

According to international law, nations can only claim waters around islands that are visible above the sea surface at high tide.

A 1987 survey showed that Esanbehanakitakojima protruded 1.4 meters above sea level, and a 1988 sea chart from the Geospatial Information Authority of Japan described it as an islet.

"There is a possibility that the islet has been eroded by wind and snow and, as a result, disappeared," Tomoo Fujii, a senior coast guard official, told Asahi Shimbun.

If Esanbehanakitakojima has sunk beneath the waves, Japan will lose 500 meters of territorial waters.

Japan also has a long-running territorial dispute with China over the Senkaku/Diaoyu islands in the East China Sea.

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