We need to remind ourselves that the mountain wilderness just north of the waters of Mormon extended from coast to coast. It had kept the Nephites separated from the people of Zarahemla for approximately 350 years. It was so rugged that a whole army could get lost in its deep canyons after only a day or two of travel. Now it was into this formidable wilderness that Alma had disappeared with his 450 Christian refugees.
It is not until chapter 23 that we learn what had happened to Alma and his followers. They actually were not too far distant -- only about 8 days travel -- but in these mountains that was enough to give them a temporary sanctuary and the army of king Noah therefore returned to Lehi-Nephi without finding them.