Honestly, I'm not sure. I thought they had a backup mesh network. I bought them because they speak in English instead of making a series of nonsense beeps; they're easy to silence when you burn something in your kitchen; they send a push alert when they need batteries instead of needing to chase down a random beep in the house; ect, ect.

I really don't care about a backup mesh network as long as they are loud enough when there is no network.

Ah, looked it up, and it seems they must have a separate protocol that they use for interconnect: https://nest.com/support/article/Are-my-Nest-Protects-still-.... And apparently it's even sort of a standard. Nifty :) Your comment about requiring wifi frightened me somewhat, but I guess they wanted to check Nest-specific things or something?

I honestly don't know. I couldn't get a straight answer from my realtor.

A few days later I was at a bar while their alarm system was getting inspected. I asked one of the pros about the situation and he told me that Nests require a working internet connection in order to pass state inspection, and all of the alarms have to be Nest.