Still under investigation, alas. First guess was a popped lithium battery downstairs but the fire dept has ruled that out now.
I know you said it’s been ruled out, but I sometimes wonder if I should worry more about aging lithium batteries in older devices… the oldest ones (e.g. 2000s handhelds) are now old enough to be retro-cool and therefore worth keeping for nostalgia, but they’re also a bit scary.