If you have enabled web access for a partner on this project and have allowed access to the selection sheet on the "Partner Web Access Settings" page, he may be able to see certain files on the selection item.
To check, click the "Edit" link next to the selection.
In the "Related Files" section, any file with a green partner icon (the picture of the 3 people) will be visible. To hide the file, click on the "Hide" link next to the icon.