I think that both alloys can be made magnetic and may be magnetic unless
they are annealed after working. I am just guessing but polishing should
not cause magnetism. You could have parts that are 304 or 316 that have
become magnetically sensitive by forming and not being annealed afterward.
Both 304 and 316 have similar Fe content ( 65%+). Also guessing, 300 series
ss that exhibits magenetism might tend to spot with rust stains if magnetic?
In out area there are labs that can verify the alloy for not too much $.
Look in your Yellow pages under Laboratories-Testing ,otherwise you'll be
just guessing the actual alloy. If it turned out that it was 316ss and was
magnetic , I don't know how concerned I would be about that? So after my 2
cents you just have more questions, no answer.

--
Ron White
Boat building web address is
www.concentric.net/~knotreel