moritheil said:
(emphasis mine)
Do you give away free information on where invisible creatures are?
In 3E, when there wasn't the prohibition on AoOs vs creatures with total concealment (the Sage claimed there was in the FAQ, but it wasn't in the rules), I handled it as a metagame player decision, rather than an in-game character decision - sort of like the decision whether or not to expend a luck reroll after the die has been rolled but before the result is determined.
So I'd say "An AoO has been provoked - do you want to expend your character's AoO for the round in an attempt to take it?" And if the player said yes, he'd pick a square, and roll a miss chance, and make an attack roll... and whether or not the attack was successful (or even into the right square), the AoO would be expended. And if the attack missed in the middle of combat, then the
character would never be aware that the AoO had been provoked at all.
Obviously, if the invisible creature was trying to sneak past someone and combat had not been initiated, the sneakee would be considered flat-footed and thus incapable of making an AoO... unless he had Combat Reflexes.
Which made Combat Reflexes a useful feat for a sentry, since it gave them a chance at uncannily detecting an invisible intruder - without a conscious decision on the character's part, he might lash out with a weapon at apparently empty space, only to discover that some instinct had clued him in to the invisible creature's presence...
-Hyp.