I suspect the Japanese warning is just coincidence. They will become suspicious when they intercept communications between the Hornet and Enterprise task forces during the next few weeks but I dont know of anything that could have tipped them this early.
I agree but given our ability to know the future when reading your posts, I suspect "Doolittle" rather than "Routine" is what first flashed across the minds of many readers. It sure was the case for me. Then I had to remind myself that this was most likely routine and was not the first Japanese civil defense warning I recall reading of.