Alma leaves and disappears. He clearly ended his role as the chief high priest over the church, and passed on the plates, so the people knew that Alma knew that his end had come. Nevertheless, he simply disappears. No one knows what had happened to him, but they speculate nonetheless.
We do not know which scriptural tradition is being cited here. Deuteronomy 34: 5-6 notes a much more mundane death and burial for Moses:
Deuteronomy 34:5-6
5 ¶ So Moses the servant of the LORD died there in the land of Moab, according to the word of the LORD.
6 And he buried him in a valley in the land of Moab, over against Beth-peor: but no man knoweth of his sepulchre unto this day.
Whatever scriptural tradition is being cited, it is lost to modern texts. Of this verse Sperry says:
“This quotation is an exceedingly interesting one. It is possible that the writer was attempting to imply that Alma had been translated, even as Moses was translated by the Lord. It will be remembered that on the Mount of Transfiguration Moses appeared with Elias (Elijah); according to the Prophet Joseph Smith, they conferred keys that were necessary for Peter, James and John to have in their future work. (Matthew 17; see Smith, Teachings of the Prophet Joseph Smith, p. 158.) It may be reasonably inferred, therefore, that Moses appeared as a translated being, not as a spirit.” (Sidney B. Sperry, Book of Mormon Compendium [Salt Lake City: Bookcraft, 1968], 364.)
The interpretation that Moses was translated appears to rest on Deuteronomy 34:6, where is says that “he buried him,” with the “he” in the verse apparently referring to the Lord. If the Lord is doing the “burying” then it is reasonable to suppose that Moses death was in the Lord’s hands. Of course one of the ways that this might have happened would be through translation. The ending phrase of Deuteronomy 34:6, “but no man knoweth of his sepulcher unto this day,” suggests that he did not have an earthly burial.
This verse in the book of Mormon appears to follow that interpretation because it notes that Alma might have been “buried by the hand of the Lord, as was Moses.”
It should be noted that there were variant textual traditions for this passage. The Masoretic text that is behind our current Old Testament preserves the “he buried him,” which appears to be the version behind Nephite scriptures. There was a different reading in the Septuagint and in the Dead Sea Scrolls, however, that read: “they buried him.” This variant makes to appear that Moses death was of a more normal nature (Martin Abegg, Jr. Peter Flint, and Eugene Ulrich. The Dead Sea Scrolls Bible. Harper Collins, 1999, p. 195).