I once came across a book with a rather interesting theory: that the body of Alexander is still around, but in St. Mark’s cathedral in Venice, venerated as the remains of St. Mark.
The line of thinking went from the fact we have reports from the Mausoleum complex surviving virtually untouched all through 365 AD, when a terrible earthquake in the eastern Mediterranean brought down much of Alexandria, and we have Libanius’ last mention of the body being venerated in 391 AD.
Now, by coincidence, the earliest report of the martyrdom of St. Mark do not mention the actual burial site of the saint’s relics after they were rescued from the pagans who were about to burn them; and once these were made, the “tomb” fell in a location that was close (perhaps too close) to the now ruined mausoleum complex. Furthermore, there are also accounts that mention the body of the saint, contrary to tradition, was actually burnt (Chronicon Pascale.) Needless to say, the earliest accounts date from the late 4th century.
Short story long, and though I do it no justice, the book holds that someone might have switched the body and miraculously produced the remains of “St. Mark” if only to spare them from destruction.