There is no indication of how this information was obtained. It would appear that some scouts were left to guard the rear of the Limhite retreat party, and that they would be able to know when they were pursued, and when the pursuit was called off. We are not told why the tracks were untraceable after two days.
The area they were passing through would be mountainous forest, and perhaps the soft undergrowth did not easily mark the signs of their passing. While there may be reasonable natural explanations for why the party could not be traced, the Lord must have had a hand in covering the tracks, as a whole city's worth of people, animals, and goods (they had brought their gold, silver, and precious things - see verse 12) would leave a fairly obvious trail.
It is also possible that rather than not being able to follow the trail, the Lamanites simply decided that they would not find them within a reasonable distance, and turned back for distance as much as failure to find tracks.
The last possibility is that the failure to continue to give chase to the Limhites was due to the discovery of the priests of Noah, which is described in Mosiah 23:30-31. These verses specifically note that it was the expedition that was chasing Limhi that found the priests. Whether they found them and therefore cut off the search for the Limhites, or were lost, and then found the priests of Noah may not be known with precision, although they do appear to have lost their way (Mosiah 23:30) and so the most probable conclusion would be that the army both lost the Limhites, and lost their way. As they were attempting to return they found the encampment of the priests of Noah.
Textual: This is a separate chapter in the 1830 edition of the Book of Mormon.