In the over 1.5 years I’ve been using Xiaomi / Aqara devices I’ve never had the batteries die, but I can confirm that their Status can be marked as “OFFLINE”, even with SmartThings’ Device Health feature turned off:
I have read reports of Device Health being unreliable, so I have it turned off.
Really the best way to know whether a Xiaomi / Aqara device has gone offline for any reason is to look at the time of the most recent battery percentage report.
All battery-powered Xiaomi / Aqara ZigBee devices check in with the hub every 50-60 minutes and send a battery voltage report at the same time. So I would suggest setting up a piston with logic that sends an alert if no battery percentage event has been generated by the device in over 2 or 3 hours.
If having trouble setting up a piston with that kind of logic, there is also a very nice customizable SmartApp which can send notifications based on time since the last XXXX event, or battery level, or temperature:
EDIT: It would be really helpful if you could check the events lists for the final battery voltages / percentages just before those sensor’s batteries died, and share those values.