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.
There's a couple of things here. First, imagine that the combat has been raging for several rounds when you suddenly burst into a room whose inhabitants knew nothing about any combat.
They could be surprised, because even though it is not
the first turn of combat, it is
their first turn of combat, therefore the effects of surprise for those guys is that they can't move or take an action on
their first turn, and are vulnerable to auto-crits from Assassinate until they notice a threat (which should be fairly soon if the warriors burst in but may be a while if they snuck in).
Second, 'surprised' has TWO effects:-
* you are vulnerable to auto-crits from Assassinate all the while that you don't notice a threat, and are
not vulnerable to auto-crits all the while that you
do notice a threat. If you start combat surprised then you become 'un-surprised' when you notice a threat, but as soon as you notice a threat (whether at the start of combat or later) then you can't become surprised while that combat rages, i.e. while in combat time/initiative order
* you may not move or take actions on YOUR first turn of this combat (whether or not
your first turn is
the first turn) and cannot take reactions until your first turn is over. Note that once your 'surprise' has mandated that you can't act etc. on your first turn,
this remains the case whether or not you notice a threat before your first turn. If you start combat 'surprised', then notice a threat (because the attacker/assassin runs into the open) before your first turn, you still can't do anything on your first turn even though you are no longer surprised!
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.
Er...yeah, exactly! Assassinate doesn't
define surprise, but it only works on someone who
is surprised at the moment the attack hits. Therefore, one of the
effects of 'surprise' it that you are vulnerable to Assassinate.
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?
As explained above,
if you were surprised at the start of
your first round of this combat, then you suffer the 'cannot act' penalty on your first turn, and that penalty has already been caused at that point. That 'cannot act' penalty doesn't go away if you notice a threat in the meantime; no rule says that! But you are only vulnerable to auto-crits if you are surprised (i.e. haven't noticed a threat) at the moment that attack hits you. Why? Because no rule says that 'being able to react' = 'no longer surprised', but 'surprised' does mean that the hit auto-crits if the attacker has the Assassinate ability.
Do you see how this is circular? "You are surprised for as long as you are surprised."
Yes, I'm stating the obvious. That doesn't make it untrue.
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
No. Just because surprise was determined at the start doesn't mean it isn't true later also.
'Haven't noticed a threat' = 'surprised. Therefore, if you
have noticed a threat then then the condition for 'surprised' has not been met. Since the default condition of creatures is 'not surprised' then there is no excuse for applying those penalties.
