UPDATE: We just got an update from EA on this. The case of older purchases being added to Origin accounts is part of a test EA is/was running that pulled its info "directly from our game registration records." In other words, it wasn't accessing any data on your home computer, just information stored on the EA side. You can read the official statement after the jump.
ORIGINAL POST: Did you have a Steam account before you had an Origin account? And did you ever buy, download and install EA titles through Steam? If so, you might want to fire up Origin and take a peek at which games it says you own. Reports from users on the Day One Patch forums (via Joystiq) indicate that some Steam-purchased EA games are appearing as owned titles in users' Origin accounts.
Dead Space 2 and Mass Effect 2 are specifically named, but from what I can tell, virtually any EA game with a product key you can access via Steam will work in this way. You can also add these titles on your own if they're not appearing. Simply highlight the game in your Steam library, click on the CD Key link at the right side of the page and input that key in Origin, under the Redeem Product Code menu option.
Users who had their Origin game list auto-filled with games they have installed are reporting that Steam still launches automatically if the game is opened through Origin. It looks like you can independently download these owned games through Origin, however, which may cut out the Steam necessity. I added Dragon Age: Origins, Dragon Age: Origins - Awakening and Dead Space 2 -- none of which are installed on my PC -- and all are listed in Origin as "Ready to download."
It seems that this process of adding games via their product keys doesn't work in all cases. I specifically tried with both Crysis and Bulletstorm, both EA-published titles, and neither one's product key was recognized in Origin.
Source: Day One Patch
Here's EA's statement:
A few of you noticed some of your older EA games showing up in your Origin My Games library over the past few weeks and I wanted to clear up any confusion. We tested a feature that in some cases made your older games show up in your Origin My Games Library. At no time was any information on your computer accessed, all of this information was obtained directly from our game registration records. Hopefully we didn’t cause too much confusion for anyone. We’ll be sure to come back and share more information about this feature as soon as we can.
Meanwhile, if you do want to activate and download older EA games to your library, we have a tool that allows you to do this. Just go to http://www.origin.com/activate and enter your registration code. Not all games are supported at this point, but we’re continually working to make older titles available. For more information on this service, as well as a list of games that are currently available for activation, see: https://help.ea.com/article/