This is a feature request for the console to know when a device is officially offline. Currently, for a cellular device, the status of the device is not known and many of our devices are battery operated and shut down to conserve battery.
It would be awesome to be able to send a flag to particle saying the device is offline due to going to sleep etc.
Just to add to your idea, how about allowing us to specify when that device is expected to wake up again. Then the device could be considered in sleep mode for that duration and if it doesn’t come back online at the expected time then it could be in an error/offline state.
Recently I’ve been thinking of building a system like this internally, like having a watchdog device that monitors and reports the status of all my other devices. But I feel like this is something that should/would eventually be built into Particle, perhaps through the rules engine, so I haven’t given it too much thought.
Check the console now. 
We just rolled out reliability improvements for that online/offline indicator in the Console. Those changes included adding a new indicator for the Electron. We are currently working towards a more public announcement with additional details.