Japanese aircraft and submarine attacks in Bay of Bengal started according to the U.S. Joint Intelligence Committee. Daily summary dated 3 April 1942

An item reported that the expected Japanese aircraft and submarine activities in the Bay of Bengal begun. Japanese aircraft attacked two ships around 200 miles south of Calcutta and on 1 April was a Japanese plane sighted 10 miles east of Trincomalee, Ceylon which was an important British naval fuel depot. Japanese submarines were further to the south and west active. Two attacks were reported 240 miles west south west of Cochin, India in the Arabian Sea and near the Lacadive Islands.

Source

Map Room Papers (Roosevelt Administration), 1942 - 1945. U.S. Joint Intelligence Committee. Daily summary No. 114 dated 3 April 1942.