OK, most alloys of stainless steel are non-magnetic. The nissan system, from the muffler (dynomax) on back is non-magnetic. The rest of the system - Y pipe and mid pipe *are* magnetic. In other words, a magnet will 'stick' to them. Is this part of the system made of mild steel? Don't know. IIRC there are alloys of stainless (low cost) that are magnetic and will get a light coating of surface rust. Is this the case? Don't know and have no way to tell.
Somebody want to pester nissuck and find out? I'm positive that the results of your inquiry would be of interest.

dave and xtoy - how much is really stainless?