Cold worked 300 Series can be slightly magnetic. See this post where I was informed how.
Thread330-26913 Austin
http://weldinginspectionsvcs.com

CF8M is not just cast 316. It has a lot more silicon for castability. Plus, as a casting, it has not had the high temperature soaking and working that wrought 316 gets, that causes the normal as-cast ferrite to homoginize into austenite by diffusion.
  The normal level of ferrite is between 5 and 20% in this cast alloy. It should not be viewed as harmful. In fact, you're essentially getting a duplex grade for free. This structure is very effective in preventing SCC. And the partion ratios of the elements work out that both phases have good and equivalent corrosion resistance.
  So, don't condemn. Spare the lowly foundryman. That's just the way the grade is supposed to be.

CF8M is an as cast alloy containing larger amounts of silicon and manganese than in 316 alloys. It seldom shows magnetic properties as compared to CF8. Perhaps the casting was heavily ground or that during shotblasting if the steel shot dust got deposited.Normally in our country sand blasting is resorted to for ss castings to overcome this problem.

The foundryman has always been the butt of all shortcomings vastly due to the ignorance of the end user in understanding the limitations of the casting process and the difficulties encountered in processing.