I don't think it would be Kevin. It wouldn't matter what time the data was accessed (between the two movies, or after Legacy takes place), since Flynn more than likely couldn't access the data while he was on The Grid, and (spoiler, sort'a) he didn't come off The Grid after Legacy (end spoiler. Sorry, don't know how to make spoiler tags). My guess is also Sam or Alan, or perhaps someone else. Someone besides Alan and Sam could have found Flynn's computer below the arcade and accessed the data, and then never said anything. Maybe it will be revealed in the next movie!