I might be able to answer that question.
In fact, there is case law dealing with section 15 in particular. I am referring to a 1990 Federal Court ruling, Information Commissioner of Canada v. Minister of National Defence. In that case, it was alleged that the department had not indicated which paragraph of subsection 15(1) applied. The Court found that the paragraph in question did not need to be referenced, insofar as one of the three areas included in subsection 15(1) was affected.