Long before the sirens sounded, Gunner would become agitated and head for shelter.[1] Gunner's hearing was so acute he was able to warn RAAF personnel of approaching Japanese aircraft, up to 20 minutes before they arrived and before they were detected by the rudimentary radar systems available at the time.[3][6][7] Gunner did not behave the same way when he heard Allied planes approaching; he could differentiate between the sounds of the engines used by Allied and Japanese aircraft.