Speaking of large numbers of people disappearing at once, have a look at this:
The Lost Sandringhams
http://www.historic-uk.com/HistoryUK/HistoryofEngland/The-Lost-Sandringhams/
The Lost Sandringhams
http://www.historic-uk.com/HistoryUK/HistoryofEngland/The-Lost-Sandringhams/
What happened to the Sandringhams during the disastrous Dardanelles campaign in the middle of their very first battle, on the afternoon of August 12, 1915? One minute the men, led by their commanding officer, Sir Horace Proctor-Beauchamp, were charging bravely against the Turkish enemy. The next they had disappeared. Their bodies were never found. There were no survivors. They did not turn up as prisoners of war.
They simply vanished.