See screenshot below. Even when I'm not using it standby drain is a joke. I have Android system set to not run in the background. Somehow still using 5 percent. I run device maintenance at least 3 times a day. If it's a rogue app it would show up in device maintenance. I'm convinced it's not as good as advertised.
First of all turn your Android System background capabilities back on, turning it off does more harm than good.
Secondly a rouge app service running in the background will not show up as an "app" in the battery usage menu, apps battery usage only show up when they are directly running.
This is a problem I have always had with Android and it leads to a lot of confusion for typical users.
You need to go through EVERY SINGLE APP and turn off its background data usage and notifications unless you need them. Do not turn it off for essential processes and services (like android system, email, sms, etc).
Turn it off for all games, bs little news apps, editing apps, etc...
Then you can accurately gauge your phones battery usage.
Example I have an app called iFunny.
After downloading it my battery started draining quickly.
Looking at battery usage it never showed that app was using battery but Android system usage went up.
I went to that app and looked. In one day it had used over 200mb of background data.
Once I turned it off and also notifications my battery usage went back to normal.