Looks like blemish in the paint. Seems to me the reason for stainless in this case has more to do with corrosion resistance than with astetics. Stainless doesn't always get a bare finish.
Yep, magnet is not a reliable way to tell. Not all grades of stainless are non-magnetic.
I like HMI, seems they've been sold though. We have a 70 ton HMI two operator unit. It's very smooth, much nicer then the 50 ton Pirahna we have. Only thing I like about the Pirahna is the automatic clamps on the bar and angle shears.
Biggest downfall to a stainless table top is that spatter will stick much more readily to the surface. Getting spatter off a stainless surface usually requires a grinder/sander, it typically won't knock off with a chipping hammer or chisel. Since you got the piece of stainless for free, maybe...