The Halo 2 and 3 database entries included links to Steam Community pages for each game, according to Stumpokapaw and Redditor whatsmyPW, who provided the above screenshot. The Community links no longer resolve those pages, and the Halo 3 database history shows the app was edited today to remove the website name.
Halo 2 and 3's existence in the database doesn't mean these games are coming to Steam, but it could point to some form of Community support on Steam in the future. With the Steam Box on the way, anything is possible (yes, even that, you absolute freak). Other games in the database, listed by Stumpokapaw, include Fez, Shadow Warrior Complete, Dyad, The King of Fighters 13 (and 2002 Unlimited Match and '98 Unlimited Match), Halo: Combat Evolved, Quantum Conundrum 2, The Witness and Second Life.
Dyad, at least, is definitely on its way to Steam, developer Shawn McGrath tells us.
Update: Microsoft says, "We currently do not have plans to release any Halo titles on Steam," in a statement to Eurogamer.