Maybe you installed a "bad" book. A single book can cause an ereader to behave weirdly, including power drain. Have you recently put a new book on it? Delete your recent books. If need be,delete all of them. See if that fixes it.
I think mine is draining because it's trying to sync all my shelves from my huge sideloaded library. I'm gonna have to keep wifi off at night.
I had the same issue and the solution for me was switching to a 3rd-party sleep cover. Problem. Solved.

Before that, the fix I used was to turn the Forma off and I never had an issue with excessive drain when I did that; the only problem was the 30 second startup.

The 3rd-party sleep cover is a better solution.
Were you using a Kobo sleep cover and replaced it with a third-party sleep cover or was the third-party sleep cover your first sleep cover for that device?
For the record, and as someone pointed to this thread, the fix for the problem that @Gadgetguy described in the first post was in firmware version 4.25.15875.
My Kobo Clara HD with official cover {only once} ran the battery down within a couple hours mysteriously.

No idea why as I didn’t knowingly do anything different that day.

I did consider that maybe the battery level indicator was wrong or not refreshed or something but not sure as never happened again.
Both my Forma and Libra have experienced a battery drain from full to zero overnight on several occasions. Both have the Kobo official sleep cover.
Try keeping the sleep cover open and using the power button to put them to sleep. You'll know if the problem is the cover or not.