Can monsters just vanish?

[Ebon]

I remember that I was playing a single player game. I was a sorcerer somewhere in dungeon level 2 or 3. There were lots of skeleton archer types grouped together in a large hall, all shooting at me. Very many. I had to take cover, and the curious thing is that when I returned to kill them, they were gone! All gone.

Can monsters really disappear? Is it an overflow?