Without an event, there won't be a trigger. But that should only matter if the device is already in that state when you create the rule (which looks like an abnormal situation so hopefully isn't the case!).
If you're concerned about the device "falling off" for some reason, there's an app for that. 
Or a few. I wrote one, which I think is the only one being maintained anymore: [RELEASE] Device Activity Check - Get notifications for "inactive" devices. However, there are other options, or if yours reliably reports specific actual events and not just "last activity," a rule could certainly work. (But if the temperature normally hovers around the same value, depending on your device's reporting configuration -- with some actually user-configurable and others not -- it may not actually send/generate events on the hub.)
