Dead Battery? 5X drained completely today in about 9 hours.

Hi folks.

As of today, my Fenix 5X has developed a problem where the battery is draining rapidly. And I mean RAPIDLY. A charge last night, put it on this morning. Did one activity (a 40 minute treadmill session). By 8pm I went to check my watch - it was dead. Tried to power back on - nothing.

I plugged it in, and the battery was showing 0%!

I've since charged it back to about 50, but it's losing charge very quickly still.

Have tried the new beta firmware to see if it helps - no difference.

Have unpaired ANT devices - no difference.

Have removed all activities from device itself (in case one is corrupt) - no difference.

Not sure what on earth brought this on all of a sudden.