Why Japan pressed ahead at Midway

Why did Yamamoto and Nagumo expect surprise at Midway despite the risk of being sighted?

They believed they still had strategic surprise and felt they had to gamble offensively before the United States could build up further strength.

Midway was a risky attempt to force a meeting engagement and regain security before American strength grew further. The Japanese command believed it retained strategic surprise and expected little American strength around Midway even if aircraft from the island detected the approaching force. Remaining on the defensive would only give the United States time to build up, so the planners felt compelled to roll the dice offensively. That assumption helps explain why sightings and warning signs did not produce the adjustment later observers might expect, without making the decision any less dangerous.