And for battery devices a wakeup can be days apart so it can be weeks before you discover a battery device has gone offline.
Link an Item to all the Channels for the device and watch events.log for ho often each updates. Then, use the Expire binding to mark it as offline if it misses one or two of those updates by more than a certain amount of time. See [Deprecated] Design Patterns: Generic Is Alive.
I use this to monitor my whole house power sensor and my smoke alarms. Though my smoke alarms have a nice and reliable heartbeat they post on a regular basis which makes it easy. You might need to just average the number of updates per hour(s) and double it for your Expire time.