I have the original Pixel as well and have see the same issue. I don’t believe it’s the security update tho. (I too reset my phone after major upgrades or problems so I don’t think that will help either.)
I can charge my phone up to 100% before bed…unplug it… and in the morning sometimes it will be at say 96% (or so), and sometimes it will drop down to 60%! (this is with the phone off)
It never does this during the day when I have the phone on.
Only at night… when my apartment can get very cold (no heater going on at night).
When first picking up the phone in the morning to turn it on it can fell like a little block of ice because of the metal back. The issue interestingly does not seem to happen on warmer days and nights. The battery will keep and hold the charge fine then.
Do you by chance have the phone overnight in a cold environment?
Also, as said by @SpeedingCheetah, our Pixel batteries are showing their age. I’m sure that is not helping our issues.
So, I’m betting it’s old and cold batteries. (just my thoughts)
If you do the Factory Reset and find it helped… let us know!