Fenix 5 - massive battery drain in a few hours

Hi, 

2yr old Fenix 5, been fine until recently, had a couple of occasion where battery is drain to 0% within an hour or two.  First noticed a coulpe of weeks ago, did reset, also updated s/w to v25, seemed ok for a week or so, but then last night it went from 100% to dead overnight.  No new settings/apps/widgets in recent months.  Happens when not doing any particular activity.

Backlight at 10% already.  Watch does track step count and movement through the day but always has and alst 1-2 weeks normally.

Any ideas appreciated!