A Japanese submarine was patrolling the area at the time. The American fleet had just passed. They could have radioed Tokyo had they seen it earlier. The outcome of the war could have been different.
In the 1976 film it was presented that a flight of USN planes spotted a lone Japanese ship steaming hard. They deduced that it was it trying to catch up to the main carrier fleet and followed it. In reality it was a lone USN sub doing picket duty that spotted the carrier fleet and alerted higher ups that the Japanese carrier force was searching for the USN carrier force.