Japanese submarines started operating in the Indian Ocean according to the U.S. Joint Intelligence Committee. Daily summary dated 26 January 1942

An item reported that a Dutch merchant ship was torpedoed by a Japanese submarine in the Indian Ocean around 550 miles west south west of Medan, Sumatra, Dutch East Indies. It was the first time that Japanese submarines operating in the Indian Ocean were definite reported.

Source

Map Room Papers (Roosevelt Administration), 1942 - 1945. U.S. Joint Intelligence Committee. Daily summary No. 47 dated 26 January 1942.