If you are still 'in combat', i.e. still believe there is a threat (character) and you are still in combat rounds (player), then you are not 'surprised' in game terms.
So that would be a "no" to my question, which leaves me feeling a bit confused since you just said that surprise is not limited to the start of combat, but can actually apply throughout combat. Now you seem to be saying that if you are "in combat", you can't be surprised at all. I don't know when else it would be relevant for someone to be surprised, unless you are meaning to draw a distinction between someone who is "in combat" and someone who is merely an unwitting participant in combat because that person's enemies have all not been noticed by that person. It seems to me that such a distinction is unnecessary, not to mention misleading, since the only direct effect of not noticing your enemies is being unable to move or act until the end of your first turn. This is what is called being surprised. If you continue to be unaware of your enemies past the end of your first turn there is no longer any effect.
As a game term, 'surprise' represents your total lack of being ready for danger, and this lack has two effects (that I know of): vulnerability to the auto-crit from the assassinate ability, and the restrictions to your actions during your first turn.
I wouldn't look to the text of Assassinate to define the effect of surprise. It only tells you what effect a successful attempt to Assassinate has on someone who is surprised. The surprised person is already feeling the full effect of being surprised whether they are assassinated or not.
If you are 'in combat' with anyone, then you are ready for danger, therefore 'not surprised' even when attacked by an unknown assailant. That unknown attacker may very well have other advantages (like advantage), but you are not vulnerable to assassinate, nor are your actions restricted by 'surprise' because you are not 'surprised'.
You defined "in combat" above as having noticed, or believing there is, a threat. I agree that having done so makes it so you cannot be surprised. It does not follow, however, that once you are surprised, noticing a threat will make you unsurprised. This would in fact contradict the rule that says that if you are surprised you can't take actions or move until your first turn is over, because if you start the encounter surprised, then notice a threat before the end of your turn, becoming thus unsurprised by your reading, you would suddenly be able to take actions and move on your turn. What then did it mean to have been surprised?
The rules don't specifically mention when 'surprised' ends, but it does tell us what makes you 'surprised' (not noticing a threat) and we can infer that when you do notice a threat then you are no longer 'surprised'.
No we can't, because it would mean you are both surprised (from the threat you didn't notice) and unsurprised (from the threat you have noticed since). I addressed this reading above, but I'd like to add that surprise is neither a condition nor an effect, per se. I believe it is best defined as the relationship between a cause (not noticing any threats at the start of the encounter) and an effect (being unable to move or act until after your first turn). A surprised creature is one to which both the cause and effect currently apply.
Of the two effects, one effect is that a 'surprised' creature cannot move or act on its first turn and cannot take reactions until its first turn is over. This effect has its own effective time limit built in.
This is why noticing a threat after you have been surprised cannot make you unsurprised. You remain surprised for the duration.
But the other effect of being 'surprised' (vulnerability to auto-crits from assassinate) does not have a time limit built in to itself; it applies for as long as you are 'surprised'.
Do you see how this is circular? "You are surprised for as long as you are surprised." Assassinate does not tell us what surprise is.
Although the action restriction effect lasts until your first turn ends, this does not tell us when 'surprise' ends. It's cause and effect: the 'cause' is 'surprised', the 'effects' are 'action restriction' and 'vulnerability to auto-crit'. The 'effects' do not determine the 'cause'!
You have the effect right. The cause is not surprise, however. The cause is being unaware of your opponents when surprise is determined, at the beginning of an encounter. A creature who is thus unaware, when combat begins, is surprised. A creature who suffers from the effect of that lack of awareness is surprised.
Only one of the 'effects' wears off at the end of your first turn; you remain vulnerable to auto-crits for as long as you are 'surprised', and there is absolutely no connection between the ending of one of the 'effects' of 'surprise' and the ending of the other 'effect' or the ending of the 'cause' itself, either conceptually (reaction speed is not the same thing as 'noticing threats') or in the rules (there is no written rule that says 'surprised' ends when you finish your first turn).
Who is vulnerable to the assassin's auto-crits? Anyone who is surprised, right? But how do we know they are still currently surprised? Surely once having been unaware of their opponents doesn't make creatures permanently surprised. I know you aren't saying that. When the assassin strikes, however, the cause of surprise is most certainly in the past because surprise was determined at the beginning of the encounter, so the absence of the cause in the present does not negate the effects of surprise. They exist for a predetermined duration. A creature can be aware of his opponents and yet still be surprised because surprise is still
in effect until the end of the creature's first turn. "Surprised" is not defined by an assassin's ability to auto-crit you, but rather the ability depends on the target being surprised.
Ah, I've seen it now. Cheers! I'm new to this forum and to this XP thing.
Yes, and welcome once again!