First brought up by a reliable source here back on February 8th who's colleague was in the December screening. The source also has a "Timeline of Events" here regarding the spoilers they posted.
Now a different source of the January/February screening are mentioning Captain Marvel as well - February 15:
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Just spoke with a good friend who happened to see an early pre screening of #TheAvengers. Simply wow! MIND = BLOWN!
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#TheAvengers aliens are wow! No way! At least I know they not the Redacted, phew!
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Seems #Hawkeye may be my new favorite badass Avenger #TheAvengers oh and the #Hulk may steal the show RE: Avenger pre screening
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RE: Apparently #TheAvengers pre-screening was missing a lot of musical score and CG on some were not yet cleaned. But wow still awesome!!
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I haven't seen the Avengers yet but listening to my pal, I want to see Avengers 2 already!
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Is future Guardian of the Galaxy movie extension of #TheAvengers? Damn I wanna see Thor vs Captain Marvel do it @Marvel
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@Marvel you guys need to get Guardians of the Galaxy moving before Avengers 2!
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@Disney @Marvel we need a Captain Marvel movie to go with Guardians of the Galaxy and Avengers 2 ASAP
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@twhiddleston I hope they nominate you villain of the decade #TheAvengers loved your performance on #Thor btw
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"If Jason Bourne was a superhero, he would be Hawkeye with a bow and arrow" - LOVE IT #TheAvengers @Marvel @Disney
And check this out, there are also reports verified by CBM that Marvel's original plan for Agent Coulson was to give him cosmic powers, and even give him a son who was going to play Wendell Vaughn aka Quasar:
[QUOTE AND LINK REMOVED DUE TO LACK OF PERMISSION]
Coincidences?
Agent Coulson as Captain Marvel?
Could it be?
Not to mention the fact that Marvel Comics is now bringing the original Captain Marvel back from the dead since the 1980's back into the comics?
UPDATE: According to the same source of the February 8th information, it may possibly just be a small reference, or possible hint to the alien forces:
As far as Captain Marvel goes, I've asked and asked and asked, and was told he's not physically in the movie. I think he was brought up in our conversation as an example to make a point regarding the alien species.
Trust me, I grilled and grilled my colleague over this so much so that if I ask one more time, I would be ex-communicated from anymore info. I was assured Captain Marvel, flesh and blood, is not in the main movie, just like Red Skull, or Thanos.
Now as for any kind of Captain Marvel reference (someone here mentioned like maybe a Nega-Bands cameo which would be effing awesome), my colleague wasn't sure if there was any, since my colleague (again not a big comic person) never really heard of CM before until a friend who also saw the screening started talking about it and the other aliens in the Marvel Univ.