I noticed today that I have those issues.
Your SmartThings device xxxx is now hub disconnected 2:07
Your SmartThings device xxxx is now online 2:07
It happens only to Zigbee devices (mains and battery powered).
Each device at a different time, the smoke detector at 1:54, the window sensor at 2:09, the Ikea Tradfri bulb at 2:12…
None of the Z-Wave devices show this behavior.