I'm using Windows 10, a Dell XPS 13, and v1.4.1 of Android Studio. I have SDK Tools 24.4.1 installed as well as Google USB Driver 11. # On the phone I have USB debugging enabled and can see the device in File Explorer (i.e. documents, pictures, etc.).
But the device never shows up using adb devices, nor in Android Studio.
I've never had this much trouble getting an Android device debugging before, but I don't know if it's Marshmallow or the Nexus 6P that's to blame. More importantly, I've no idea what to do at this point to get it debugging.
I had this exact problem and solved it by downloading (http://developer.android.com/sdk/win-usb.html) and installing the Google USB Driver by hand.
Before I did that, I tried installing it from the Android SDK Manager which did not work. The package installed without error but my Device Manager showed a failure for an ADB Interface device.
After downloading/extracting the zip file from the above link, I updated the failed ADB Interface driver in Device Manager and now I can see and use my phone in both Android Studio and from the command line with adb.
I'm assuming that you have also enabled USB Debugging on your phone as detailed here: http://developer.android.com/training/basics/firstapp/running-app.html#RealDevice