I think I've finally found the reason of battery drain.

Switch off ass voice wake up command

Double click on home button to open ass voice > uncheck wake up command

I reckon that with the feature on, the phone is always on the lookout for any background sound resembling a command to wake up hence the phone is on alert all the time

After I've done the above, my battery drain on night standby was only 2% compared with up to 20% previously even with all power saving mode on n other tips already listed on the first thread