On December 27, 1429, an official from the "Office of Posthumous Titles" was sent to Kangwon Province to find a place known as "Yodo" (蓼島), a mysterious island believed to be somewhere in the East Sea (Sea of Japan). Though the official did not find the island, the search would continue unsuccessfully off and on for more than thirty years. The following is the first reference to the ghost island known as "Yodo."
Source:『世宗實錄』世宗 46卷, 世宗 11年 12月 27日 (己亥)Notice that the official sent to find the island was from Bongsangsi "The Office of Posthumous Titles," which was an office charged with deciding on posthumous titles for Chosun officials. That seems strange to me since such an official seems unqualified to search for a mysterious island. Maybe Lee An-gyeong was sent to confirm claims made by a dead Chosun official that would help in deciding that official's posthumous title? At any rate, subsequent records will show that King Sejong became somewhat obsessed with finding Yodo.
December 27, 1429
봉상시 윤(奉常寺尹) 이안경(李安敬)을 강원도에 보내어 요도(寥島)를 방문하게 하였다.
Bongsangsi Official Lee An-gyeong was sent to Kangwon Province to visit Yodo.
I have included this record in "The History of Ulleungdo" because it is possible that Yodo was another name for Ulleungdo. It is also possible that Yodo could have been a reference to Dokdo/Takeshima, but the records suggest that the island was bigger and much farther north, somewhere in the sea east of Hamgyeong Province.
The significance of "the search for Yodo" is that it shows that King Sejong, especially, was interested in finding new territory in the East Sea (Sea of Japan). Considering such interest, it seems unlikely that the discovery of any island in the East Sea would go unmentioned in Chosun records. The fact that there was never any mention of Yodo or any other island, besides Ulleungdo, being discovered in the East Sea seems to support the claim that the Chosun government did not know about Dokdo/Takeshima.