Once a faction is dead, it's dead. There's no magic re-appearance like in MTW where you crush a faction 10 times only for it to return yet again with another gold peasant army which kill all your knights. The exception to the rule are the West and East Roman rebels which pop up whenever there's a disloyal General or one of their cities' rebels. If a faction that hordes loses it's last settlement and every one of its family members and soldiers is killed at the same time, it can still horde - it's happened to me. But that's the nearest to a re-emergent faction as you get. The example of a settlement rebelling and becoming Sarmatian owned is probably just a loyalist rebellion like what happens occasionally in RTW. There must still have been a few Sarmatians on the map for them to gain the settlement. Sometimes you can't see them, but if you look at the mini-map there will be a star giving away their position if they're in horde mode.


As to the Huns appearing at a set date, er... they start the game on the map already chap. It is possible it's left over code for the White Huns which never made it to the finished game.