As with the story of the escape of Limhi's people, this preparation could not have been the simple task of preparing for the night.
The gathering of flocks might be a nightly task, but the gathering of the supplies needed for the journey would be a much larger task that any normal nightly preparation. As with the Limhites, this suggests that the Lamanite/Amulonite presence in the city was limited, and that they were able to make these preparations without raising an alarm with their oppressors.