Albert Barnes Commentary Exodus 14:5

"And it was told the king of Egypt that the people were fled: and the heart of Pharaoh and of his servants was changed towards the people, and they said, What is this we have done, that we have let Israel go from serving us?" — Exodus 14:5 (ASV)

The people fled - This was a natural inference from the change of direction, which indicated a determination to escape from Egypt. Up to the time when that information reached Pharaoh, both he and his people understood that the Israelites would return after keeping a festival in the district adjoining Etham. From Etham, the intelligence would be forwarded by the commander of the garrison to Rameses in less than a day, and the cavalry, a highly-disciplined force, would be ready for immediate departure.