Mysterious heavy battery drain during device sleep since update to M I recently updated my Note 3 to Marshmallow, using the resurrection remix rom, and I am experiencing excessive battery drain I cannot pinpoint the source of.

I am a very light device user, prior to updating to M I was using an Lollipop based mod - xnote, and my phone lasted for up to 10 days.

The drain appears to manifest randomly, I cannot tie it to anything in particular, sometimes it goes away after a device reboot, sometime it persists, it has triggered during my sleep, when the phone is completely idle, no wifi or whatsoever. I've taken care to prevent third party applications for running on boot and keeping the device away, and as the phone information shows, the CPU is in sleep 98-99% of the time. It is not the battery either, as until last week it could easily last a week, the battery itself has no more than 200 discharge cycles. I have disabled NFC, WIFI, GPS, bluetooth and pretty much everything non-essential yet the battery drains heavily while the phone is sleeping, doing nothing. Also the phone is set to "power save" settings.

Here is what I am talking about:



The image above shows battery levels from one reboot, with the drain persisting after the reboot, draining half of the battery in 12 hours of idling, as is obvious from the images below. Then shortly before 4 PM I did another reboot which removed the drain.



Here is another image, illustrating a longer period with and without the drain, absent the drain the phone loses 2-3% battery a day.



Any ideas how to pinpoint the source of the drain and prevent it? Marshmallow was supposed to improve battery life, but in my experience so far battery life with it is terrible.
I didn't manage to pinpoint the actual source of the drain, however it was finally resolved by replacing the stock kernel that came with the rom with idlekernel. Back to losing about 10% change in 24 hours on my modest usage pattern.

Update: several weeks into using the phone with that kernel and I noticed some things were not working properly. This led me to investigate the matter further, to finally arrive at the conclusion that the drain was the product of old firmware. After updating the bootloader and modem the drain has disappeared, most likely it was the modem, now battery usage is normal even without the custom kernel.