As the filming of the Guardians of the Galaxy has already wrapped up and is currently in post production, the only logical conclusion was that (OMG, this was when my jaws dropped when realisation hit me), they were a Marvel Studios film crew filming the next Marvel Cinematic Universe superhero movie - Avengers 2: The Age of Ultron! With crews and visitors in t-shirts bearing Captain America's logo and Converse's Chuck Taylor All-Star logo (you know with the big five pointed star in the middle, you didn't need to be a genius to figure it out. The symbolism was strong. It all hinted that whatever was going on in the ballroom had something to do with Captain America.
We decided to hang around a bit and chatted with the crew who were pretty tight lipped. But considering that the area was crawling with extras in 1940s military uniforms and rumours on the street was that Chris Evans (the Captain himself) and Hayley Atwell (Peggy Carter) were spotted, there is no doubt that the scene being filmed in question is a flashback sequence. A style ballroom was perfect for a period scene. Someone else also mentioned that they were filming at the local college not too far away, and that Scarlett Johansson was spotted, presumably filming a present-day sequence.