Edit: running the beta firmware 3.21
I charged the watch up last night (approx 7pm) and put it back on my wrist.
It was still working and showing the time this morning at around 5am when I woke up but it then turned itself off and wouldn't turn back on.
I plugged it into the laptop and it turned on and showed 3% charge.
I'm guessing that an activity got accidentally started or syncing got stuck or something but I have searched for files with modification times of today (only ones showing have the time of the restart) and yesterday (latest ones were some segments I synced when charging and seg_list.fit and another single fit file in the monitor folder all dated when I was connected to the laptop for charging yesterday)
Is there anything I can do to determine what happened?