It is quite difficult to reconcile the following quote from the article with the postulate requiring that the disease causing agent be present in all cases of infection. "We have only identified the corona virus in 50% of the people being treated for Sars," health ministry spokesperson Paul Gully told the Sunday edition of the Montreal newspaper La Presse."
My guess is that they just haven't tried hard enough yet. The alternative is that we are still facing something that as you indicated we know nothing about. I would love to see some more about this since my guess and $1.75 might get you a cup of coffee at Denny's.