Speculation is awesome. :)
I've spoiler tagged our perspective on this ad:
you guys nailed it - essentially this is Kat pre-arm loss - the mission in which she lost it - and presumably the original noble six you replace at the start of Reach the game. This was Bungies position, that it's not actually on Reach - but its intentionally nebulous. Casual fans won't care and will assume it's Reach, hardcore fans know it cant be. You guys are Halo CSI - can't get nuthin past you :D
Glad people enjoy it, turned out pretty sweet overall IMO.
Time for mega marketing to glass your wallets. :lol