Alternatively you could use the menu option in the Tools > File Management menu that moves files for you. Select all the games in that platform, and then use the tool to move those rom files to a different folder, games left over are your orphan files. Though it IS possible those files WERE imported, but were combined into additional apps. I'd run an audit of the platform and then sort by the Additional apps column and see if any of the files got placed under a parent game.