Aaargh, this just happened again. This is a 100% repeatability problem, in my case. It rained here over the weekend, and I guess the solar panel isn't mounted in an ideal position, so it wasn't able to charge the battery sufficiently. Battery drains fully = dead Electron.
The code I am running is linked in the previous post.
Fortunately the PoC I'm running is nearly over so I won't have to fight this much longer. I would be pulling my hair out if I had to deal with this long-term. Actually I'd probably mandate that 100W solar panels were used (rather than 10W) so there would be no possibility of battery discharge, even on the cloudiest of cloudy days.
I do hope for future users of this platform that the problem is resolved. If there's anything I can do to help Particle work out what's going on here, please let me know.