Zero didn't exist before Juni, so if Zero is in a story, it's post-Melrose.
The post-humanity Unity also doesn't have smoke detectors or a lot of other things. Their society is built around there being infinite electricity that literally comes from the sky, but relatively limited amounts of material. Only chemical reactions that are reversible via electrolysis or photosynthesis or the like are sustainable for them. All metals are precious and very carefully recycled, and plastics are basically completely out of the question. Even batteries aren't really feasible - current has to be stored in capacitors.
So that's what this story is about.
...and plastics are basically completely out of the question...