The Torah tells us Shemos 14 (5)
It was reported to Pharaoh that the people had fled; and Pharaoh and his servants had a change of heart toward the people, and they said, What is this that we have done, that we have released Israel from serving us?
And Rashi comments
It was reported to Pharaoh: He [Pharaoh] sent officers with them, and as soon as the three days they [the Israelites] had set to go [into the desert] and return had elapsed, and they [the officers] saw that they were not returning to Egypt, they came and informed Pharaoh on the fourth day.
Why did the officers wait for the whole three days to elapse? It would take the Bnei Yisroel some time to return to Egypt if that had been their intention. The officers could have returned once it was clear that Bnei Yisroel would not arrive back before the three days had elapsed.