We have been getting bikes in the shop with this problem a lot lately. Stainless spokes that get small rust spots on them. They break right at the rust. Look like they were cut but weren't. We have had people try to blame their ex for doing it. I blame it on cheap, poor quality, steel.
If you have the rust spots, I would change them all or in some cases replace the wheels.
Dolan Kadet , Cannondale R600, Cannondale M900, '85 GT Pro Performer
Do you store you bike in the vicinity of chlorine (swimming pool chemicals), lawn/ plant fertilizer or water softner salt? If so, that is your culprit. In an enclosed space such as a garage or storage shed the off gassing from these chemicals starts to attack small imperfections in the steel of the spokes, usually at or near the crossing junction of the spokes. If that is the case, the wheel is toast. You can keep replacing spokes, but they have all been affected at this point. You'll just keep replacing them one after another as they fatigue. Sorry news is you need to buy a new wheel, or at least relace the spokes and nipples to your existing rim and hub.
I work at a shop in Florida and all three of these chemicals are quite common and almost always stored in garages. The heat and humidity here (90+ degrees/F and 80%+ humidity) just speed up the process. We get a couple wheels a week with this problem, a record of 5 last week.