I disagree with the idea that ALL attacks a preceded by clues which are automatically perceived, even by creatures that failed their Perception checks. It's against the rules (in that successful checks mean you gain the information, while FAILED checks should mean you FAIL to get that information) and contrary to the established narrative.
Let me take it step-by-step. First, there is more than one way to be 'not surprised'; I'll just talk about the two ways that we have been discussing:-
1.) 'notice a threat' (by way of the DM saying you notice it automatically, or a successful ability check contest)
2.) 'immunity to surprise' (by way of the Alert feat, Weapon of Warning, various other abilities)
In the same way that 'poodles are dogs' AND 'rotweilers are dogs' does NOT mean that 'poodles are rotweilers', you can be 'not surprised' because you 'noticed a threat', you can be 'not surprised' by being 'immune to surprise', but this does NOT mean that being 'immune to surprise' makes you 'notice a threat'!
Being immune to surprise means that you are 'not surprised' even though you did NOT notice a threat!
If you are a trained assassin, spending all your resources on being almost undetectable before you strike, and you make all your skill rolls, use silence/invisibility and so on, and the victims all fail to match your Stealth with their Perception, but they happen to have faster reflexes than you, the fact that they are immune to surprise should not result in the DM ignoring the facts that the rolls have established re: you being hidden! The DM should not give information that should be gated behind successful Perception checks, nor should he narrate your arrow 'hitting or missing' (which is what breaks hiding, not 'attacking') before you resolve your attack.
Sure, the victims have a chance to detect the attack before it is resolved (hear the bow creak as it is bent back, or whatever), but we already rolled for that and established that the victims FAILED to notice these clues. Therefore, the DM should not give these clues anyway!
The alert guy in this situation has NO direct information about the (successfully hidden) assassin!
The alert guy DOES know that combat is starting, and can make educated guesses to help him make smart decisions.
Immunity to surprise is very useful, but it is not an automatic 'I win' button; it does not make you 'notice a threat'; it makes you 'not surprised' even if you FAIL to 'notice a threat'.
Still digging through the thread, but this is a great post.
While I'm not a fan of any rule that guarantees success (I prefer advantage on surprise situations, but RAW doesn't have a surprise check), the most important thing here is to not get caught up on the "meaning" of the word surprised within the game rules.
What is surprised?
It means you can't move or take an action on your first turn of the combat (which in 3.x would have been called flat-footed), and you can't take a reaction until that turn ends.
The "immunity to surprise" is really more about reflexes than it is about being surprised. So somebody with the alertness feat is simply faster at reacting to the unexpected. It's not really about educated guesses, it's about training your body to react without conscious thought in my mind.
For the musicians out there, think of it like learning to play a fast line in a piece of music. Your fingers move faster than your brain. This is the same thing, the body acts instinctively.
An example:
One of the few uses of a reaction is the shield spell. So the assassin is hidden and undetected. We'll say he's using a bow. Normally, the conditions are perfect, the assassin is undetected, and he takes his shot. Because they are surprised, they are unable to react. But somebody with Alertness picks up the slightest clue - a glimpse of the arrow from the corner of their eye, sort of an innate sense, and reacts, casting their shield spell.
Yes, the assassin was undetected, and remained so. But as soon as the arrow was loosed, there was something new to detect. Most people miss it altogether, but somebody with alertness picks up and reacts instinctively.
And bows don't creak. But that does allow a real life example. Bow hunters typically don't like their chances from anything more than about 30 yards out. One of the reasons why is because the bow string (and other pieces of equipment) does. There are mufflers available, but it still makes noise. And a deer has amazing senses and reflexes, and that is enough to make the difference between a kill shot and a deer being injured with an arrow. The deer could have been standing with 10 of its human friends, and it would be the only one that would have reacted - without even thinking about it. That to me is alertness in action.
