It says engaged in combat, not melee combat.
The very same sentence then continue talking about the "Character has Superior Position"-modifier which is clearly a melee combat modifier (that you cannot take during ranged combat).
It is clear that this sentence applies when defender is already engaged in combat and a melee attacker is:
- sitting over the defender's back in a grapple.
- attacking from behind.
- attacking from above.
"Defender already engaged" in "Defender unaware of attack" reference "Character has Superior Position" because this part only apply to melee combat situations. If it would also apply to ranged combat situations then they would have also referenced "Shooting through Barriers" (but they didn't).
It is not clear that it also applies to ranged combat.
SR5 p. 197 Barriers - Shooting through Barriers
If an attacker wants to shoot through a barrier and hit a defender on the other side, a few things need to be determined. A defender using the barrier as cover receives a defense bonus for cover. 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. If the barrier between the attacker and defender is transparent, like bullet resistant glass, there is no cover or obstruction to sight, but the attack must penetrate the barrier to reach the defender (see Penetration Weapons, p. 198).This rule is talking about ranged attacks only. In this case "the [completely] hidden defender is considered unaware of the attack". Full stop. The only exception to this is if the cover is transparent, in which case the defender is able to see the attacker and therefor is no longer unaware. There is no "This does not apply to defenders in combat"-exception to the rule as we could read in the Defender unaware of attack rules when they were talking about the Character has Superior Position
melee modifier. When it comes to
ranged attacks it doesn't matter if the target is engaged in combat or not. What matters is if you can see the attacker or not. If you are completely hidden behind a barrier (or shot in the back) you don't get to take your normal defense test.
unless you have some other way to "see" them...
Such as "hearing, smell, touch, and even taste".
We are going around in circles.
I personally think you should suffer the Blind Fire modifier if you can only "sense" the subject of the spell without actually "seeing" the subject, but in this case it doesn't seem as if RAW agree with me.