I disagree with your interpretation that characters "always" gets a surprise test; while this is true in cases where the GM decides that a Surprise Tests is called for, the GM is not actually required to call for surprise tests at any point according to the rules. If you wish to include surprise when a character ducks behind a wall and completely leaves line of sight for opposing parties, that's up to you, but it is by no means required by the rules as written.
Page 192: "The surprise rules below apply to all situations, whether
all the parties involved are caught off guard or whether
one or more parties are intentionally ambushing others."
All combats begins with surprise. According to the "surprise rules below," (192) there is no situation in which a character automatically fails to use its Defense. It only loses its ability to apply a Defense Roll if it fails an Intuition + Reaction (3) test. If it is being ambushed, at best, the ambushers have no chance to be surprised. Often, the ambushers must attempt to roll surprise as well, they just get a +6 die pool bonus. If someone makes their perception penalty, all they gain is a +2 die pool bonus to their surprise test (192-193). By the RAW, a GM is required to roll a surprise test to initiate ALL combats. Additionally, by the rules as written, it "is possible for [surprise] to happen within a Combat Turn if an unexpected force enters the fray."
In fact, by the rules as written (page 197) the hiding character is either
A) using the barrier as cover, in which case the attacker takes no penalty and the defender gets a +4 dice roll modifier to his defense test (as per the Defender Target has good cover rules on p190), or
B) is completely hidden behind the barrier, in which case the attacker takes a -6 Blind Fire penalty (page 178) and the defender "is considered unaware of the attack"
This is not strictly true. By the rules as written, page 197 does say, "If the defender is completely hidden behind the barrier, the attacker suffers a –6 Blind Fire dice pool modifier for not being able to see his intended target, but the hidden defender is considered unaware of the attack." But, the rules also say, on page 190, "If the Defender uses a Take Cover action to get behind something where more than fifty percent of the defender’s body is obscured by intervening terrain or cover, he gains a +4 dice pool modifier to his Defense roll against any attack. This modifier can also apply to prone targets that are at least twenty meters away from their attackers. This modifier is applicable to both Ranged Combat and Spellcasting.
Note that this modifier does not negate the Blind Fire modifier the attacker suffers. Both the modifiers to the attacker and to the defender would apply when firing at a target that is totally concealed (one hundred percent behind cover)."
The rules EXPLICITLY note that BOTH modifiers apply. Thus, at first glance, page 190 and 197 contradict each other. Hence my point of contention. Some sort of rule must exist that makes BOTH page 190 or 197 true; otherwise, if we are going to simply say that one of the pages is wrong, the most logical choice would be to say that page 197 is the page that is inaccurate. That is because page 197 is the page that creates a greater degree of implausibility. Personally, I would like to find some way to make both statements true, which means using
another rule that clarifies what being "unaware of an attack" signifies in a way that doesn't lead 190 and 197 to contradict each other.
So to answer your original question, being unaware means being considered to have already failed your surprise test.
Look at the Defense Modifiers on page 189:
"Defender unaware of attack:
f the defender is unaware of an incoming attack (he does not see the attacker, the attacker is behind him, or he is surprised), then no defense is possible. Treat the attack as a Success Test instead. This does not apply to defenders who are already engaged in combat (see Character Has Superior Position, p. 187). If the defender is behind cover, the defense dice pool is determined by the cover, according to the Defense Modifiers table."
You are correct. Those are the rules on page 189. But again, the game contradicts itself. First of all, the surprise rules clearly indicate that NOBODY is ever depleted of their defense roll UNLESS they fail their surprise test. Furthermore, the rule you are quoting EXPLICITLY states that "it does not apply to defenders who are already engaged in combat." Such defenders should instead see the "character has superior position" rules on page 187. Furthermore, even if the character is not yet in combat, this rule states that a character still makes use of his cover's defense dice pool.
So, lets see what it says under page 187: "Whether it’s sitting over an opponent’s back in a grapple, attacking from behind, attacking from above, or
attacking from stable ground while the enemy fights to stay standing, having the better position is an advantage in any fight. Whenever the attacker has a significant advantage in footing, perception, or mobility, grant this +2 dice pool bonus."
So, by the RAW, you seem to be wrong. It seems like, according to a
very strict reading of the rules as written, this is what happens: If a character is 100% behind cover when you attack them, and combat has not yet begun, we run into a contradiction of code. On one hand, the surprise rules say that everyone should roll surprise. At best, the person making the attack gains the advantage of Ambush on their surprise check. The barrier rules seem to suggest that the foe is automatically unaware of the attack. The most fair interpretation seems to be that the attack counts as an Ambush, but if the target is not surprised then we move to page 187 and use the superior position rules. Alternatively, I could see situations where an attacker is unseen and opens a fight with an attack as resulting in a defender automatically being "unaware of an attack." Either way, during combat, we always move to page 187 and use the superior position rules (as being unaware of an attack always turns into superior position during a combat encounter).
Which means, during combat, if a target is 100% covered by a barrier the attacker takes a -6 penalty to attack, gains a +2 bonus to the attack, for a net -4 penalty to the attack. The defender may use their defense roll, and they gain a +4 bonus to that defense roll. Outside of combat, if the target has 100% cover by a barrier then the attack either a) results in a surprise roll on which the attacker has the benefit of Ambush or b) a is true, but one way or the other the hidden attacker results in the target being considered unaware of the attack
if the attack is the first event which opens up combat (as otherwise the characters are all considered to be in combat, and the "unaware of an attack" rules default to the "superior position" rules). Either way, these rules seem like a mess to me... errata and clarification seems to be needed.