It has the option to notify you when there hasn’t been any activity on the device for whatever period of time you set. Most devices report in at least once a day, some more often than that. So “last event” won’t be just when you last used it. It’s a good stand in for off-line.
Because these are mesh devices, there isn’t the equivalent of an off-line report like you would get from Wi-Fi device. The devices are just supposed to check in occasionally, and the absence of a check in will then represent being off-line.
For more questions about it ask in the authors thread, and people there will be able to tell you specifics.
Note this in particular:
Receive Push and/or SMS notifications based on temperature, battery level, and/or time since last event.