If that was the case wouldn't battery usage, under the app. section, show the app. in question?
I guess one sure way to find out is to restore, then set up as new. I'm just trying to avoid that for obvious reasons.
Back in iOS7 day's you couldn't see that. Since iOS8 you should see it there indeed. If it can't be seen there then it could be caused by all the reasons as given in my post earlier (e.g. location settings, e-mail push/fetch, iCloud) which should be also shown at battery usage (normally).
I restored the phone last night and set it up as new. Will see what happens today.
Let it charge over night, the usage and the standby is still weird.
It seems fine again. That problem seems to be resolved then.