I've never seen appliance lost power/reboots reported in Dashboard network event log, nor in the events returned by getNetworkEvents (using V1 API).
It always seemed a curious omission, as on the device page summary tab, the 'connectivity' timeline bar does pop-up a message saying there was a reboot and the cause (typically lost power or firmware update), clearly the information is there.
I haven't tried, but my guess is webhooks or SNMP would be the best bet, as I think they can give alerts for device going offline/online, you'd need to set up something to receive them.