What part of the Feeds api knows if there is a difference between old and new data?
I am modifying the code in hook_feeds_before_update and writing the code like this :

if ($item){
    $node = node_load($item);
    $changed=false;

      if ($entity['ITEM'] != $node-&gt;field_item[LANGUAGE_NONE][0]['value']){
         if (trim($entity['ITEM'])=="UNKNOWN"){
             $entity['ITEM'] = $node-&gt;field_item[LANGUAGE_NONE][0]['value'];

         }

      } 


AND then I flag it with a rule :

{ "rules_flag_when_updated_by_feed" : {
"LABEL" : "Flag when updated by feed",
"PLUGIN" : "reaction rule",
"OWNER" : "rules",
"REQUIRES" : [ "flag", "feeds" ],
"ON" : { "feeds_import_csv_importer" : [] },
"DO" : [
  { "flag_flagnode" : {
      "flag" : "flag_asset_for_review",
      "node" : [ "node" ],
      "flagging_user" : [ "site:current-user" ],
      "permission_check" : "1"
    }
  }
]


}
}