I gather it's a coin flip when you are doing "Return to Sender": Either the station is still operational and under NCR control, or else when you arrive to deliver the new codes, everybody ( -1) has been killed by the Legion. In the latter case, there is a recording left by the raiders that ends with, "...we took one of the women alive".

Is it possible to chase down the raiders and rescue the captive? This could be one of the untitled sub-quests -- which means that you won't get any clues on the World Map to show you where to go. Or it could just be a "bit of color" meant to send the player scurrying hither and yon on a fruitless wild goose chase. [There IS a rather large Legion raiding party laying in ambush near the campfire just East of Ranger Station Charlie -- but that may just be a coincidence.]

Anybody know definitively which it is?

Ranger Station Charlie only gets raided after you take to Ranger Andy about the station. Its a trigger so as long as you don't engage it, it won't happen.

Edit: Whoops only read the first part. Anyways,

There are Legion nearby Charlie, before and after it happens, as you mentioned. However, I haven't met a woman in either situation.

Don't know about captive but there is quest for retired ranger in Novac. He will ask you to go check on his comrades at station. You have to visit him twice, at first he will not give quest to you (may be you need some reputation in Novac before).

I forgot all about the captive taken from Ranger Charlie but found this on the wiki:

Listening to the Audio Logs, there's a warning from the Legion to the Rangers, also explaining that
one of the Ranger female soldiers was taken as a hostage after the raid (this ranger can later be fought
as an opponent in the arena at fortification hill).

Interesting...

Oh that's who that ranger was.

She was tough as hell.

I would have loved to rescue her ... first time round went to everyone who had a say in the NCR after having found the tape but noone referred to her or seemed to be interested ... hunted down every Legion near and far from the location without luck - also read about being able to fight her in the arena - though as a sniper and Boone-Fan i never got to that event.
Kind of frustrating like the situation with the boobytrapped soldiers between Nelson and Forlorn Hope but there you go.

She is at fortification hill even without having Ranger Station Charlie massacre happen . She put up a fight, but a level 12 killed her(me xD).

Hmm. Does she appear at Fortification Hill _only_ if you follow a pro-Legion path? I don't recall seeing anyone in the arena when I answered Caesar's "summons" whilst following a pro-NCR/Independent path.

She only appears when you speak to the guy beside the arena, saying that you want to fight her.

In which case, that means there is absolutely no way to save her. The only interaction you have is to kill yourself rather than letting the Legion do it.