How to investigate battery drain issue on iPhone?

I'm investigating a battery drain issue. I previously thought it's the hardware but now I'm actually more leaning towards a software issue. However, I'm struggling to find out the root cause.

Here is the history in (very short) bullet points:

• started experiencing this around mid April
• did some investigation, didn't find anything
• went to an Apple store for diagnosis - battery ok, phone was resetted, and recovered from iTunes backup
• issue seemed better at first but after a few days it was clear it wasn't -> opened case with Apple -> phone was resetted and recovered from iCloud backup
• battery drain was not fixed -> phone has been replaced by Apple
• same issue on new phone after restoring from iCloud backup
• thus, I went through the pain and reset the new phone again, starting as a new fresh device (including my watch, also started from scratch)
• issue was fixed for a while but since yesterday I have the battery drain again 😟

There are a few confusing observations

• no new app has been installed in the last few days when the drain reappeared on the new phone
• no new app had been installed back in April then the drain first appeared on the old phone
• no change in setup was done in the last few days
• no change in setup was done back in April then the drain first appeared
• monitoring the battery statistics on the phone, it seems that
  • active time is fare higher than I actually used the device

    it does not match the standby time, though

  • the sum of times of the visible apps does not add up to the actual active time

    where is the other 5+ hours spent? is it in the system?

• the phone is pretty warm when I pull it out of my pocket - even though I haven't used it for two hours.

What I tried so far:

• reset, restore and update from iTunes backup
• reset, restore and update from iCloud backup
• new device
• dozens of restarts of phone and watch
• disabling push for everything in mail, contacts and calendar
• disabling background activity for most apps

Given the observations in the battery statistics, I'm wondering if it's some system service that is draining the battery.

I am suspecting it's the watch. Thus, next thing I'll try is unpairing them then going without my watch for a while.

This is the setup:

• iPhone 6s (iOS 9.3.2)
• Watch (watchOS 2.2.1)
• One Exchange account with mail, calendar and contacts (Google Apps Active Sync. provisioned via device management Airwatch MDM)
• One Gmail account with mail, calendar and contacts (personal)
• iCloud photos, Safari and find my phone (drive disabled due to device management profile)

Any help is appreciated!