what I would do is enable DEBUG as explained here: [piwik.org]
then generate a visit from RATP (in a new browser, to trigger a new visit) and look at the piwik.php output. Is the referrer mentionned? is there an error?
then generate a visit from RATP (in a new browser, to trigger a new visit) and look at the piwik.php output. Is the referrer mentionned? is there an error?