Currently the Homekit Controller when integrating the Nest Protect accessory from Homebridge only adds Smoke and Motion entities, whilst the Protect support CO as well. I can see the CO characteristic is available in the information received from Homebridge, but the integration doesn’t consume it.
I think it would be fairly easy to add support by duplicating the Smoke entity, which seems to utilise the HA binary sensor DEVICE_CLASS_SMOKE, however there is no DEVICE_CLASS_CO in binary sensor. I’ve seen conversation about using DEVICE_CLASS_GAS, but not sure of any downstream affects of just duplicating the smoke code.
If I get a chance, I might give it a go, but would welcome any advice. At the moment I can’t see in the code where it is filtering out unwanted characteristics, so any pointers would be welcome 