echo what Scott just said, I engrave a ton of stainless (went thru seven 500gm bottles of Cermark last year!) and the only rust issue I can remember was from sanding transfer as I noted above....the same sheet metal shop sanded some aluminum sheets that were black anodized, right after sanding a ton of stainless, lots of silver streaks and small embedded flecks of stainless all because of careless sanding...

I'm thinking it's just a fluke.

As for what may work as a quick fix- get some Naval Jelly (phosphoric acid) and some Magic Erasers. Both are safe to use on stainless, the NJ will take care of any rust, and the ME will help do it quickly! Wear gloves, rinse well and dry...

re: Magic Erasers- while these things will quickly scratch acrylic, and just breathing hard seems to scratch stainless, I use them even on mirror-polished stainless with no scratching or hazing at all. Very aggressive use might, but just basic wiping, wet or dry, they work great. And you CAN get a bit aggressive with brushed stainess, just go with the grain.