I wanted to report I'm having the same issue here with an iPhone 7 plus, and iOS 11.1.2. I have managed to get the percentage to go down to 82% from 97. The only reason I knew there was an issue is because yesterday the phone had gone from a full charge to low in just a few hours sitting idle. Usually i get almost a week with my usage of the phone before it's time to charge again, give or take my usage amount of course.
I did a hard reset of the phone, and I've turned off "listen for Hey Siri" in Settings:Siri & Search. There doesn't seem to be an option to completely disable Siri that I've found in iOS 11.
I did want to mention, that my iPad Air 2 has the same configuration as the iPhone 7 plus, and does not have this issue. The only difference is the iPhone has the ability to listen for Hey siri without the need to be plugged in.
I don't generally use bluetooth to stream audio, but do use Google Cast instead as I have a couple of sound bars that support it. In fact, the only use of Bluetooth I have on those devices are to pair the Smartcast app with the sound bars to control Volume and settings from the app. there again, I don't think that's an issue as only one device lists Siri in the battery menu. I do have dictation turned on, but I don't use it all the time, and there again, it's only the iPhone with the strange problem.
If anyone has any other suggestions beyond restoring the phone, which I can do. I would appreciate it. Thanks