The most embarrassing way to expose Kido Butai

What would be the most embarrassing way for Kido Butai to be detected?

A radio-equipped civilian liner accidentally sailing through the formation despite the route's careful avoidance of shipping lanes.

The most embarrassing discovery would be an ordinary civilian liner with a powerful radio sailing directly through the formation. Japanese planners had deliberately selected a route far from normal shipping lanes, so the encounter would signal a major navigation or staff failure. The liner could transmit a warning quickly and would leave Nagumo with the problem of a neutral civilian witness. Detection by precisely the traffic the route was designed to avoid would turn an operational-security success into a conspicuous blunder.