Pharaoh was told "that the people had fled" (Exodus 14:5). But had Israel actually fled? The Torah itself states in (Numbers 33:3) that "on the morrow of the Pesach, the children of Israel went out with a high hand" — boldly, publicly, in broad daylight. That does not sound like fleeing.
The Mekhilta resolves the contradiction with a vivid scene. Pharaoh had stationed emissaries to monitor Israel's departure. But the Israelites attacked those emissaries — beating them, killing some, and wounding others. No one stopped the violence. No authority intervened.
The surviving emissaries staggered back to Pharaoh with their report: "Look, Israel beat us. They killed some of us and wounded others, and no one stopped them." They described the Israelites as a chaotic swarm with no visible ruler or officer, citing (Proverbs 30:27): "The locusts have no king, and they all go out in a single swarm."
This is why Pharaoh was told the people "fled." It was not an accurate description of Israel's departure — it was the panicked report of beaten men. From Pharaoh's perspective, his emissaries had been overwhelmed by an ungovernable mob that answered to no one. That perception of chaos and defiance is what convinced Pharaoh to mobilize his army and give chase — a decision that would lead him straight into the waters of the Red Sea.