When a device is unreachable by HE, please change the state to unavailable and prevent it from being used in apps (similar to how SmartThings deals with it).
Context:
I recently had a battery on one of my Iris v2 motion sensors run out. There was no indication from HE about this and the motion sensor got stuck in "active" mode with no way to mark it as "inactive". As such, my home never went into "night" mode (which occurs when there's no motion everywhere except the bedroom) and my thermostat never changed. Also, I had no way to know why it was stuck so was wondering if there was something wrong with the hub.
In SmartThings, when a device can't be communicated with, the device gets marked as unavailable and I believe it is no longer used in any of the automations. I've had the exact same situation (battery run out) in SmartThings and my home still performed as expected (as though I didn't have that motion sensor) and I knew what the issue was. IMO, that's a much better way to indicate that there's something wrong with it. What would it take to implement this in HE?
One problem is not all devices report battery level accurately (if at all). Then, as you noticed, some devices fail in what appears to be an active state . . . how do you deal with that. I understand the problem but I'm not sure the cure isn't potentially worse. I suppose if it were configurable (so I could opt out) it would be OK. Also, you may want to reconsider the logic for your mode switch . . .
As I was responding, I had this feeling that it had been a bad experience in ST . . . I couldn't remember the details until you posted the words "Health Check". That was a nightmare . . . fortunately a long time ago.
I'm not sure how ST did it but this specific devices would show up as "unavailable" instead of staying "active". They must have been doing something similar to the Device Watchdog suggested above.
What's wrong with my mode switch? Within a certain time period and if the mode was already in "Home" (so we're home), if there's no motion in any room other than the bedroom for an hour, then turn on night mode. When night mode turns on, thermostat goes to sleep mode, living room TV and lights, kitchen, den lights are all turned off. Used this for years in ST and worked great. Personally, I think motion should be one of the ways to set modes in Mode Manager.
Not sure if it's the same thing (I suspect it is) but it's called "Device Health" on ST. If it is, didn't realize it caused so many issues. It worked great for me e.g. when a hue bulb was connected to a switch and the switch was off, the bulb would show up as "unavailable". HE would still turn the bulb on/off and show the updated status even though that wasn't the correct state of the bulb
There was quite a lot of discussion on the ST frum about it.
Caused issues for me also.
I had it disabled all the time. Every time there was an update it would enable itself again.
Battery reporting is mostly a miss on every platform I've been on but device watchdog works great for devices with a temperature sensor and a few battery reporting devices. Most devices won't report a battery level until it changes so it could be a week or more until they check in again and some won't report until something else wakes it like the first alert smoke/co sensors seem to sleep until a test is run or smoke is detected, whereas some like Schlage locks checkin every 8-12 hours. Devices that do temperature reporting usually do it when the reading changes enough or every 1-6 hours so I have device watchdog report every 14 hours but leave off the few devices that don't report in enough like light switches, bulbs, Osram Dimmer switches, and a Yale lock. All of my contact, motion, water leak, buttons, Schlage locks are in device watchdog.
