What was the “movement among the people” that Noah saw? From the context, it was not merely a change of opinions and religious beliefs, but rather a physical movement—probably the hundreds of people who were quietly disappearing from the city. Some disappeared permanently, while others came and went. Noah ordered spies to trace those movements, and they followed them to the land of Mormon or perhaps posed as believers to infiltrate the group while in the city.
Much later, Mosiah 23:1 specifies: “Now Alma, having been warned of the Lord that the armies of king Noah would come upon them, and having made it known to his people, therefore they gathered together their flocks, and took of their grain, and departed into the wilderness before the armies of king Noah.” Since the believers still had access to their herds and storage, Alma’s preaching did not create an immediate and irrevocable separation from Lehi-Nephi. It was obviously the believers who came and went between Lehi-Nephi whose “movement” Noah observed and whom the spies followed.
It is also obvious that they were conducting missionary work in Lehi-Nephi, or their numbers could not have grown from two hundred to four hundred and fifty in a relatively short time. Whether overtly or covertly, those who had accepted the Messiah returned to Lehi-Nephi and found others who were willing to accept the Messiah. These new converts accompanied their missionaries to Mormon where they were baptized into Yahweh’s church.