Bumping this thread since I have the exact same wish. Zigbee is a two way protocol…so I imagine that it would not be that hard to implement some kind of “ping” from Homey that checks if a mains powered device is “live” or disconnected from power and therefore from the zigbee network.
Just today I have replaced an old incandescent bulb in the shower outside our sauna to a IKEA Trådfri. The reason is that I want to be able to have the bulb dimmed when we are using the sauna (there is very dim light in the sauna and when you get out to take a cool shower a bright light is straining on the eyes and breaks the cosy feeling). I also want to have bright light in the shower for cleaning it and other situations when we are not using the sauna.
What complicates this is that the light in the shower and in the sauna is on the same wall switch, so turning off the shower light using Homey will keep the sauna light on. This means that I have to use the wall switch. Why not changing out the sauna light to a Trådfri to you ask? Well, Trådfri is only rated to +40° C…and that would not be much of a sauna would it? 
In general I feel that Homey lacks indicators for whether devices are “online” or if they have lost their connection to the network. Another example is sensors. I have a multitude of thermometers, light sensors etc using the 443 MHz protocol and there is no way to know if they are connected or not. The only indicator is to check when the last value was reported.
Since Homey only reports when values changes there is no way to know if the (say) temperature is 100% stable for an hour or if Homey have lost the communication with the sensor, maybe because the battery has gone flat. This annoys me immensely! I constantly wonder if the value in Homey is correct or if the sensor is sending in the blind to Homey. This about the sensor got a bit OT…so I think I will start a new thread about this. 
