An early sighting could have postponed Pearl Harbor

What would happen if the Kido Butai were spotted en route and Nagumo followed his orders?

An early sighting could call off and postpone the operation, while a same-morning sighting would probably be too late to prevent launch.

The answer depends on when the Kido Butai was detected. A sighting well before the attack could trigger Nagumo's call-off message and postpone the operation for months. Japan might later attempt Pearl Harbor again, but delay could also give Yamamoto time to reconsider the force commander. Detection on the morning of the attack was much less likely to stop aircraft already positioned to launch.