Sam, it is not exactly accurate that all T316 is austenitic. If I remember correctly, it is the materials like Chromium (Cr) and Mo (I use the symbols cause I can't remember how to spell it!
) that give stainless steel their resistance to corrosion, and Ni and Mn to lower ferritic properties. When stainless steel is cold worked, it can transform to be ferrite, by transformation of martensites (sp?). Thus while the magnet test is a test lots of people use to sort 300 series steel, it can be wrong.
That said, it seems that Nauticam had QC issues with the quality of their stainless steel parts. The magnet test, while possibly inaccurate, is a good test, since the alternatives like spark and acid aren't exactly going to work! 
In fact, recently I did such a test on the Nauticam D7000 housing and found several parts to be ferritic, including the locking mechanism. I subjected the magnet test to the Seacam and Subal D7000 housing and all points were austenitic. Now as I said, the magnet test doesn't necessarily mean anything on its own, but with CP's statement about the lower grade steel being used, it could mean something and owners would do well to check their housings more vigilantly, especially those with housings that are older as this problem would probably manifest later in the the life cycle.