This is an interesting situation because the watch still unlocks your Macs, which tells you the watch itself is working fine and it's specifically the iPhone pairing that's broken. The Bluetooth connection being visible on the iPhone but the watch showing as disconnected is a classic sign that the logical pairing has broken at the software level, even though the underlying Bluetooth link is still partially there.
To answer your two questions:
On how to re-pair without a full factory reset: Unfortunately there's no clean way to do a partial re-pair. The intended process is to unpair the watch from the iPhone (which creates a backup automatically), and then pair it again and restore from that backup. This is less destructive than a full factory reset because your health data, watch faces, and settings are preserved in the backup. To do this, open the Watch app on your iPhone, tap the i button next to your watch name, and choose Unpair Apple Watch. Once unpaired, put the watch near the iPhone and the pairing prompt will appear.
On what causes it: The disconnection during a flight is almost certainly the trigger. When you enable Airplane Mode, the watch and iPhone both drop their connection cleanly. But the issue happens when Airplane Mode is turned off and the reconnection process doesn't complete correctly. The watch can get stuck in a state where Bluetooth shows connected but the Watch app doesn't re-establish the full link.
To avoid it happening again after flights, try turning Airplane Mode off on both devices at the same time, then wait a full minute before using either. If they don't reconnect automatically, put them close together and open the Watch app on the iPhone.
The battery drain when disconnected happens because the watch keeps searching for the iPhone connection continuously, which burns through power faster than normal.
