Mysterious heavy battery drain during device sleep since update to M
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.