I played through ME twice (paragon and renegade alignments) and loved both times.  I sold my copy and a few months ago got the itch to play again so I bought a used copy.  I started it up and was surprised to find both of my old saves still there.  Thing is, I am really anal about deleting my saves from my HD once I sell a game and I know I had deleted my ME data.   
 
Where did this come from then, and will it happen again when I boot up ME2 (I hope, I hope)? 
 
Thanks

There are two types of saves. One is the end game save, and the other are regular game saves. I think you just deleted the regular game saves.

there are some game saves that stay with your gamertag. I've recovered my profile on a friend's xbox and had my saves transfer to his box as well as my profile... I don't know the specifics of it all, but that's probably what happened here

" There are two types of saves. One is the end game save, and the other are regular game saves. I think you just deleted the regular game saves. "  

ME uses two save systems, your "game save" and your career save. Your game saves are the ones you can "see" and your career saves are the ones you can only load up when you start a new game. They happen when you beat the game and I don't think you can delete them by conventional means. 

Well, I created an infiltrator and beat the game with him and I can no longer swith to that career. I did nothing to delete it, and now that I have a new xbox he is not there.

On another note, those saves are generally unmovable from the console, so if you want to move a concrete Shepard to another Xbox in ME2, you need to use the endgame save.