Battery lost 9% overnight

Yes, and even a lot higher (about 20% at max). And I did not even wear it overnight. E.g. I placed it on a table beside my bed. Therefore I started turning the device off overnight. It happened more than once that I noticed a very low battery the next day just before I wanted to go out for a run...
I suspect it has something to do with the BT connection to the app (my smartphone was downstairs but still in range of the watch).

@Stuquires As per another post I made earlier today I am losing about .62% per hour - that's over a 67 hour period including 1.5 hours indoor swimming . I do find with the 5+ that when I charge it to 100% that it often hasn't charged to 100% - so on Friday I charged to 100% restarted the watch and it showed 89% I had to charge for another hour before it would show 100% after restarting (Obviously I don't know if the 100% is right even at that point). Try next time you charge to see whether you are fully charged or not. Is it the 5S or the 5+ that you have

Thanks Heidstocker - I will make sure that bluetooth is off this evening. JSRUNNER_ I've just "turned it of and back on again" and before and after it read 88%. It is charging now, I'll try again when it gets to 100%. (it is the 5S+)

StuSquires ok will be interesting to see. Unfortunately battery is quite small on 5S so life is even shorter than the 5+ as I'm sure you are aware. As regards BT connection not sure what impact that has - I have mine on all the time but my phone is by the bed at night so the connection is good and I don't see much drain overnight. I wouldn't want to turn the watch off as part of the point of it is 24/7 tracking

JSRUNNER_ Charged to 100%, reboot, still reads 100%. Does the watch poll for a bluetooth connection, or does it just wait to be asked? If it does poll, can that be turned off?

StuSquires if the BT connection is turned on then yes it will poll. You can easily turn it off https://www8.garmin.com/manuals/webhelp/fenix5plus/EN-US/GUID-139CD52F-B25F-4002-B13D-ACF7FEA70A04.html but I have found BT has very little impact on battery (I haven't tried it however in your situation with phone downstairs).

On the charging front I am now at 38% after a further 70 minutes GPS running last night so that's about .67% per hour now. I do use vibration alerts both in training and during the day these I believe use more power than audible alerts and I have not tried turning these off. At my present discharge rate I guess I may make it through my wednesday evening run before having to recharge.

The battery indicator on this thing is hilarously jacked up. I charged it 24 hours ago, went for a short run using the GPS last night (about 3.5 miles).. and this AM, the battery indicator is still at 100%. Granted this watch is only a few weeks old, but c'mon Garmin. Weird how it's also been almost 3 months without any kind of firmware update to a 6-7 month old watch... hope they're working on a big patch for this thing!

rtperna I have to reboot mine to be sure its 100% charged in the first place - on friday I had to charge for about another hour after it had first showed me 100% before it would show 100% after a reboot. Also over the first 7-8 hours the percent displayed doesn't drop from 100 but if you reboot you'll find that it has. They are working on this I know. Not sure what you mean about upgrades - I have had a few since getting the watch in October. Which model and SW version do you have - I am 5+ on 6.51

rtperna They are working on this I know.

Can I ask how you know? They seem to be taking an absolute age to release these bug fixes.