When the data navigator renders the XML of an object old version it includes information about the (eventual) invalid references that have been dropped from the object - e.g.:

  <img nav:objectType="img" id="obj6494"  . . .>

    <nav:versionCreation>

      <nav:actionDesc Action="Update" Time="2011-05-19T23:24:53.137Z" LoginGuid="c1a62a83-3fe6-40a4-9c05-786251273749" 

        UserId="1705" UserDesc="TeraDP\MiMo" /> 

    </nav:versionCreation>

    <nav:creation>

      <nav:actionDesc Action="Create" Time="2011-04-03T15:55:19.987Z" LoginGuid="7decb101-42cb-4b4d-9b0d-e4c0d988480d" 

        UserId="1705" UserDesc="TeraDP\MiMo" /> 

    </nav:creation>

    <nav:modified>

      <nav:actionDesc Action="Update" Time="2011-05-19T23:24:04.863Z" LoginGuid="c1a62a83-3fe6-40a4-9c05-786251273749" 

        UserId="1705" UserDesc="TeraDP\MiMo" /> 

    </nav:modified>

    <nav:invalidRefs>

      <nav:attr n="img.folderRef" ids="6815" /> 

    </nav:invalidRefs>

    . . .

  </img>

is the XML of an old version of the 'img' object with id 6494 that referenced the folder with id 6915 - the reference is no longer valid (the folder has been deleted), and so it has been dropped from the object XML.