I think it was back in 2011 when Bungie were celebrating their 20th Bungie Day, they released an hour long video talking about Bungie over the past 20 years. At the end of the video there are hints to a new game, I believe they title it "Crimson" or "Tiger", can't remember exactly. But that was the first time I had an idea they were working on something big. I didn't hear the title "Destiny" until the 50-odd page legal document that was released by Activision when they were in a legal dispute between some members at Infinity Ward. And here we are, Destiny is only a few months away! How time flies, eh? :P