Currently a static filter is able to only retrieve contents linked to a site.
Visibility of content (private, public, shared) is checked when the context is "all sites" or a list of sites.

To be able to retrieve contents outside a site such as ODF contents or user contents, we need a "no site" context.

Be careful, this filter should only retrieved contents with no site property, otherwise it will be easy to by-pass content's visibility setting context to "no-site"