Right after the invasion of Pearl Harbor, the newspapers often had headlines like this such as in San Francisco and Los Angelos. Reports were confirmed by the military but there was never any proof of the enemy planes. After the war, Japan confirmed that it never flew over the coast of California. If you look at the history books and newspapers from that time frame you would see that Japan was very busy in the China Sea with major offensives. Wikipedia has an interesting write-up about this called Battle of Los Angeles where they say it might have been weather balloons.