What brand? What gauge stainless?

A high quality 16 gauge sink should never rust. My Franke 16 gauge sink is over 10 yo. Looks the same now, as it did when installed. No rust.

It is a 18 gauge sink

Don't condemn the sink quite yet. A tiny iron particle may have settled in the sink from the factory or even when the installation was preformed. Or a drop of corrosive chemicals used in tile grout, countertop sealers or other products could of gotten on the sink. This "temporarily" strips off the sink's protective chromium oxide layer.

In both cases it can be cleaned off without leaving any damage. Use a non abrasive oxalic acid cleaner (look it up and take your pick) to rub it off in the direction of the grain with a non abrasive cloth, not a greene pad, scratch pad, sandpaper, steel wool or anything that will scratch the steel.

If it doesn't come out.....well I don't know what you should do then.

The odds are overwhelming that what you're seeing is topical contamination. Clean it off; it's unlikely to return. Check the area for steel scrubby pads and other sources of metal, please.