I have 22 nest smoke detectors and using the 1.9 binding with openHAB 1.8.3. I thought things were good since I get back green and ok from every detector, but today I was blowing the dust off my batteries in my electric room and got an early smoke alart. The alart was announced on my other detectors and I got a push notification my my nest app on my phone, but in looking at the openhab logs everything was still green and ok. 
Hi, by coincidence I also had an early warning last week, the nests performed well, push notificatio from the nest app, but no warning from openhab. I did not give it much thought (it was in the living room, where I was actually when it happened, we were not paying attention to the cooker, and I was surprised the nest detected the smoke, it was really a very very early warning). Anyway, your question triggered me to have a look in the openhab logs. I noticed indeed openhab was informed, so I should create a rule to act on that. Here are the relevant entries in my log
2017-03-15 16:08:44.687 [ItemStateChangedEvent ] - NestProtectLivingRoom_state changed from green to yellow 2017-03-15 16:08:44.706 [ItemStateChangedEvent ] - NestProtectLivingRoom_last_connection changed from 2017-03-15T15:48:40.509+0000 to 2017-03-15T16:08:28.973+00 00 2017-03-15 16:08:44.710 [ItemStateChangedEvent ] - NestProtectLivingRoom_smoke changed from ok to warning ... 2017-03-15 16:10:50.524 [ItemStateChangedEvent ] - NestProtectLivingRoom_state changed from yellow to green 2017-03-15 16:10:50.540 [ItemStateChangedEvent ] - NestProtectLivingRoom_last_connection changed from 2017-03-15T16:08:28.973+0000 to 2017-03-15T16:10:13.235+0000 2017-03-15 16:10:50.547 [ItemStateChangedEvent ] - NestProtectLivingRoom_smoke changed from warning to ok
For completeness, my items are defined as follows:
String NestProtectLivingRoom_state "Status Color [%s]" { nest="<[smoke_co_alarms(Living Room).ui_color_state]" }
DateTime NestProtectLivingRoom_last_connection "Last Connection [%1$tm/%1$td/%1$tY %1$tH:%1$tM:%1$tS]" { nest="<[smoke_co_alarms(Living Room).last_connection]"}
String NestProtectLivingRoom_smoke "Smoke Status [%s]" { nest="<[smoke_co_alarms(Living Room).smoke_alarm_state]" }
String NestProtectLivingRoom_co "CO Status [%s]" { nest="<[smoke_co_alarms(Living Room).co_alarm_state]" }
String NestProtectLivingRoom_battery_health "Battery health [%s]" { nest="<[smoke_co_alarms(Living Room).battery_health]" }
String NestProtectLivingRoom_time_since_last_connection_String "Last connection [%s]"