Sabrecats and wolves almost always spawn and hunt in groups of 2 or more. They can spread out a decent amount - particularly the Sabrecats if one spots prey and kills it before the other one turns to join in - as they seem to be able to spot and react to things from farther away. I've noticed that when one of them spots you, the other one or two will almost always come running to attack you as well unless they are too far away. This means that for sabrecats, compared to wolves, you are more likely to encounter and kill one by itself but also far less likely to have seen the other one crouching off in the grass somewhere till it circles around behind you and attacks.
That gets to NPC sneaking. NPCs don't really sneak like you do, but the animal AI seems to include sneaky behavior for some of the creature types. While playing in 3rd person mode I have noticed that sabrecats and wolves coming to attack you while you are fighting another of their kind will actually circle behind you and come at you from behind. This means that you often won't ever have seen their red dot on you compass and from first person perspective they seem to appear behind you.
They appear to have a crouched movement animation (and are low profile already) so in places with high grass they do blend in really well sometimes. Its possible that there may be a more general NPC sneak mechanic as well - it might not hide them from your direct vision but maybe on your compass. I'm not entirely sure about that one though.
I think it does take some liberties. For instance, even when muffled and under a 60 second invisibility spell, you'll find that NPCs will still say, "I thought I heard something!" Uh, how in the name of Jove did you hear anything when muffled means you move silently?
With sneak I think that has a lot to do with why, when escaping Helgen, the brief introduction to stealth which occurs at one point includes the line "Watch where you step" or something along those lines. When I first started playing I was getting noticed all the time for things I hadn't considered mattering because stealth in their previous games basically rendered the AI entirely braindead when it worked. First time I really noticed those bone-chimes hanging from the ceiling in a lot of dungeons was when they ruined my day when I thought I was getting the drop on a draugr deathlord I was about to get the drop on and a bit too weak to fight under normal conditions. Also figured out a little later in that same dungeon that sneaking with a readied glowing and roiling ball of flames in your hands is perhaps not the most effective choice if you plan on approaching undetected. When this works it does make stealth more interesting, but at their behavior can be very inconsistent. Also, setting off runes placed as traps in the dungeons is strangely treated by their detection rules as YOU casting the spell and therefore attacking. That one should have probably been handled differently - sure they'll come running and be more alert but they should not immediately detect you upon triggering.
The only thing I've noticed personally that does feel like NPCs cheating though is the Jet-Li style arrow dodges they seem to pull off once they're aware of someone shooting at them. I want that move.