Diablo – Are Übers and Key Wardens considered elite monsters

Most enemy spawns are completely random.

The only types of special enemies that spawn deterministic are event bosses (events and sidequests with a "mini-boss" will always have one, although not necessarily always the same) and main-story quest-related monsters.

But since events themselves are randomized, in and of themselves they are not worth focusing on.

It is the case that some areas seem to have a higher density of elites and other high end mobs. Their spawns within an area are not deterministic per se, but if you clear the zone you will tend to find more in some areas than others. One particular area of note that fits this mold is the Vault of the Assassin in Act II.

It's triggered almost all the time you encounter a Rare/Champion pack is visible. I say almost all the time because I have noticed instances where my follower has not announced them. I am yet to determine whether that is due to an internal cooldown from him talking previously or if he simply does not announce it 100% of the time.

I have also noticed a single time where he used one of his sayings usually reserved for after a fight with a Rare/Champion pack after a normal battle, as this threw me (I was not expecting it). However this was post-fight, so should not affect your concern.

In conclusion, their sayings are a good way to indicate a Rare/Champion pack is near, but their silence may not mean an absence of said packs.