I have the remains of a number of spokes with the identical problems. I'm surprised you see this under the circumstances you describe because it's characteristic of chlorine damage. Search for chlorine damage in stainless steel and you'll find lots of links describing the phenomenon. Basically the chlorine ions enter the steel's crystal structure and render it weak and brittle.

On my wheels, spokes would snap with a pronounced ping while the bike was just sitting there. Mine better fit the profile in that these bikes were in Cozumel, and exposed to sea water and salt air 24/7/365. When I examined them the spokes were full of inclusions (3-5 per spoke) so I had to rebuild both.

I was first made aware of the issue by my dentist who hoped I could help him solve a problem with sterilization of his tools. The autoclave (steam) dulled them quickly, but repeated sterilization in Chlorox made them brittle and tips would break off without notice, something that could be serious in his line of work.

Anyway, most grades of stainless used for spokes are moderately resistant to chlorine damage, but some spokes made in Taiwan a few years ago were quite vulnerable. These made their way into the OEM pipeline ended up on a number of new bikes (including the pair I sent to Mexico) a few years back. I'm surprised to see the problem is back.