If a character attacks someone and the target is not aware of their presence, does the target lose their Dex bonus to AC?

Question

If a character is hit by an enemy and they are not aware of the presence of the enemy prior, like getting hit by an enemy by an arrow from behind, do they lose dexterity bonus to Armor Class for that attack?

Answer 1

Awareness as a term of the rules is checked only at the start of battle. In this case the rules of a surprise round apply.

If awareness is thought of in a broader sense, the rules on hiding might apply after the start of battle.

Rules Compendium p.92:

If you're successfully hidden with respect to another creature, that creature is flat-footed with respect to you. That creature treats you as if you were invisible.

That means: Yes, if the character is unaware of the enemy, because the enemy is hidden, that character loses his/her Dex bonus to AC and the enemy also gets a +2 to hit.

This general rule can be used in melee ("Sneaking up from Hiding") and ranged attacks ("Sniping").

Rules Compendium p.92:

Sneak up from Hiding: You can sneak up on someone after emerging from a hiding place. For every 5 feet of open space between you and the target, you take a -5 penalty on your Hide check. If your Hide check succeeds, your target doesn't notice you until you attack or perform some other attention-grabbing action. Such a target is treated as being flat-footed with respect to you. Sniping: If you've already successfully hidden at least 10 feet from yur target, you can make one ranged attack, then take a move action to hide again. You take a -20 penalty on your Hide check to conceal yourself after the shot.

So, essentially, if the enemy is successfully hidden and closes in on the character with yet another successful hide check, he/she benefits from being hidden with his/her first attack. This implies denying the Dex bonus to AC and +2 on attack ("as if you were invisible").

This rule even seems to imply the enemy could follow up with more attacks and still benefit from being hidden in case he wins an opposed check with -20 on Hide:

You can also take a -20 penalty on your Hide chek to hide while attacking, running, or charging.

This is admittedly hard to imagine and as a DM I would rule against it, but the rules seem to allow it.

The sniping option allows to hide after the shot with a move action. So if the enemy manages to win the opposed check even with the harsh penalty of -20 he could shoot the character several consecutive rounds ignoring its Dex bonus to AC and getting +2 on attack.

Answer 2

In dnd-3.5e, awareness is determined from a set of senses including, but not limited to visibility (Spot). An opponent can be hidden from you, but you may be aware of their presence (and thus not surprised), usually by some other sense, normally hearing (Listen check) but can also be tremorsense, scent, etc.

From surprise,

Determining awareness may call for Listen checks, Spot checks, or other checks.

You also see this in the Spot description,

A Spot check result higher than 20 generally lets you become aware of an invisible creature near you, though you can’t actually see it.

Thus the answer to the question

If a character attacks someone and the target is not aware of their presence, does the target lose their Dex bonus to AC?

Yes, because unaware also implies unseen and the attacker gains +2 on attack rolls against the unaware defender.

In 3.5, a target is denied their Dexterity bonus to AC if the attacker is invisible and the attacker gains +2 on all attacks against a sighted defender. If you can't see the attacker, the attacker is effectively "invisible" to you.

You can also make a similar argument for blinded, but the outcome is effectively the same.

The Rules Compendium kindly makes the explicit connection for you.

If you're successfully hidden with respect to another creature, that creature is flat-footed with respect to you. That creature treats you as if you were invisible.