The Swashbuckler Charmed Life Ability states:
"... Three times per day as an immediate action before attempting a saving throw, she can add her Charisma modifier to the result of the save. She must choose to do this before the roll is made..."
Does the Swashbuckler have to be aware of the source of the saving throw to use this? IE, walking into an area with an aura that requires a save vs watching a dragon about to use its breath attack. Obviously one of those is easier to tell is about to happen, but can a Swashbuckler *always* use this ability as long as he has hit swift/immediate action available still, or is there any other restriction on this?
Does the Swashbuckler have to be aware of the source of the saving throw to use this?
Not necessarily, but . . .
can a Swashbuckler *always* use this ability as long as he has hit swift/immediate action available still, or is there any other restriction on this?
Yes, but the bolded part is rather important due to the following rule:
You also cannot use an immediate action if you are flat-footed.
"Flat-footed" applies to the time during combat before the swashbuckler acts. Does it apply when the swashbuckler is surprised by a non-creature effect, such as the aforementioned walking into an aura without knowing it's there? That's debatable. Most of the time it doesn't matter, since flat-footedness doesn't affect saving throws, but in this case it's a bit of an oddity of the rules.
You are aware of needing a save whether you recognize the source or not.
One example of this is from the magic section: "A creature that successfully saves against a spell that has no obvious physical effects feels a hostile force or a tingle, but cannot deduce the exact nature of the attack."
So a character is aware that 'something' is happening, and that 'something' requires a save, even if they don't know what it is.
That only applies after the saving throw, and then only if the saving throw is successful. If it fails, the victim is not alerted of anything unless the effect itself makes it self-evident.
Regardless, even if the swashbuckler creature doesn't know they're making a saving throw, the player should. In the case where the GM is secretly rolling saving throws, it's trickier. One method is for the GM to allow the player to set up criteria for using their ability, similar to how one might resolve the issue of when to use roll-twice-take-better abilities on hidden rolls.