Apple

The day I updated to 10.1, my watch dropped from 55% to 5% in 6 hours while I slept and on low power mode, I run one 3rd party app that installed when I got the watch 12 months ago.

It obviously drained and turned off itself, I put it on charge(and forgot about it) for about 6 hours and since then it’s been back to great battery life. So from that and what others told me, after it updates the watch is running overtime in the background doing its thing.

Hope this helps.