Us warning signs at peal harbor?

The one major warning sign at Pearl Harbor happened at approximately 6:30 that morning. As a ship entered the harbor a sailor on another ship thought he say a periscope trailing behind the ship. They were able to contact a nearby ship - the Ward - who confirmed seeing signs or a sub as well. They fired on the submarine sinking it. When the message was sent it was sent encoded that they had 'fired upon' an enemy ship. The telegram did not state straight out that there was a successful attack on another ship. If it had, if it had not been encrypted - those in harbor may have had a chance to prepare for the attack.