It can be assumed that the Lamanites had spies in the area and were aware of Moroni’s force prior to the night march. However, they had calculated what size of garrison needed to stay behind to protect Mulek, how much time it would take Moroni to reach Mulek from his last-known position, and how long it would take them to catch and fight with Teancum’s force. The night march had thrown all their calculations awry, and they were at risk before they even realized it.