I have built a very large village in my world as my primary base.  My son was playing on his laptop the other night(he is working on a project very far away from base) and was hunting creepers at night.  I was at the base, and not sleeping.  Overnight, all but 2 of the villagers were gone- while wandering the base, there were no zombies, no zombie villagers, and no doors broken down to indicate a raid.  I was able to get the 2 remaining villagers to breed a single child.  I then went to sleep normally, and when i woke up the 2 adult villagers were gone.  After one more night of sleep the remaining villager was gone.  It seems they are disappearing or despawning.