[curmudgeonII]

The implication of Herodotus and others is that this army disappeared without trace in a sandstorm. If nobody witnessed this how did anyone know the disappearance was due to a sandstorm?
Why not a depletion of their water supplies, a revolt of some of the troops [most of the troops were from subject nations] that killed most of the combatants and left the remainder disoriented or dispersed to areas where they could not make their presence widely known for fear of revenge, etc.?

[SunkenCiv]

If nobody witnessed this how did anyone know the disappearance was due to a sandstorm?

The refrigerator light goes off when you close the door. ;')

The sandstorm wasn't localized, there were other witnesses to it. The Persians never came back, and so the obvious inference was, they were killed by it.

It's still a good point, because Herodotus said it struck at their midday meal. But obviously, ancient Persians ate lunch, and the time of onset of the storm would be observed or inferred from those who could take shelter (such as those who lived at el Kharghah).