On the official site's news about Nadia's exit, there is no mention of her being a "mole". I suspect it may be a voluntary exit that many consider to be a PR stunt, as happened with Marion and Rosa in Secret Story France.
This could be a reference to her entering just to leave the next day. On the picture on the news article, the caption of the photo says "Nadia Lindén task in Big Brother 2012 was to submit an assignment"
But overall it appears she was a housemate, and just walked.