Interesting info about StSt and magnetism from the British Stainless Steel Association:
It is possible for the magnetic permeability of austenitic steels to be changed during processing. For example, cold work and welding are liable to increase the amount of martensite and ferrite respectively in the steel. A familiar example is in a stainless steel sink where the flat drainer has little magnetic response whereas the pressed bowl has a higher response due to the formation of martensite particularly in the corners.
I just tried this on my sink, and it works exactly like they said! You learn something every day.