I use Wakelock to find the most draining apps and then use Greenify to suspend them. Specific versions of ROM and specific versions of apps have draining issues, if I find I'm in that problem of the ROM draining then I go back to the prior version, if an app is bad then I go back to the prior version or uninstall.
The most commonly draining app not related to usage is Facebook.
But for example this last week the Uber app went into drain mode, I was in airplane mode on an actual airplane and I'd used the the Uber app before going into airplane mode and it was frantically trying to find out what to do next. I picked up a very warm phone, then noticed, then killed Uber and plugged in power.