Some time ago, I remember reading where in the leadup to the raid, Doolittle reassigned men perceived as "talkative" to the most remote, desolate bases possible.
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.
The Japanese article mentions the possibility of an attack from China or Alaska.
Several weeks ago I ran into one of the Ploesti Raid B-24 pilots and he said that their original target was supposed to be Japan using bases in China, but they were diverted to Egypt to hit the Romanian oil fields.
The Japanese might have gotten wind of this B-24 plan, or they might just have been looking at a map!
It is ironic that right after their own surprise attack from carriers on Hawaii, they wouldn't mention the possibility of a carrier-launched surprise attack on Japan.