Nearly all stainless is faintly magnetic but it's not always obvious. 304 is faintly magnetic as a rule. If you have a magnetic that hangs let it hang very close and you'll see. It's not enough to feel the pulling force, just to see it visually. And like greytruck said it can be become more magnetic in some circumstances. Industry standard is less than 2% ferrous contamination is fine for clean stainless. After all, iron is the larger part of the stainless alloy anyway. Yards that grade it different than that are either doing it wrong - honestly or dishonestly - or they deal with a buyer that grades differently than industry standards (which is possible, for example SA Recycling has Painted Al Sheet and Al Sheet 95% instead of a single Old Sheet category because they do aluminum a little different)