The iOS 10.2 Release Notes includes this:
Adds notification support for HomeKit accessories when software updates are available to HomeKit accessories
I saw this (and captured it in a video) on one of my iDevices. A badge appeared on the Apple Home app icon. Lauching the Home app showed the message:
FIRMWARE UPDATE AVAILABLE
Update the firmware for "Device Name" in the "iDevices Connected" app.
Underneath that was a link to open the iDevices Connected app.
How is this being done? I cannot find any documentation in the HomeKit API.
Thanks!