Misleading docs lead to this report:

Steps to reproduce:
1. Open the attached project.
2. Connect an Oculus Rift to your PC.
3. Press play.

Expected:
In the console you will see the boolean value VRSettings.isDeviceActive printed every frame. According to the 2017.1 documentation, this should be true when "Unity is accepting input from the HMD AND rendering to the HMD's display(s)." (https://docs.unity3d.com/2017.1/Documentation/ScriptReference/VR.VRSettings-isDeviceActive.html) Thus, this value should be false when the proximity sensor detects the headset is off the user's head because the HMD screen stops rendering.

Observed:
When taking the HMD off the head, VRSettings.isDeviceActive remains true.

Brought to our attention by in 2017.1.0f3 by Unity forum post (https://forum.unity.com/threads/oculus-rift-proximity-sensor.525014/).
Further tested and found present in most recent 2017.1 patch (2017.1.3p3).
Further tested and found present in most recent 2018.1 beta (13)
Further tested and found present in most recent 2018.2 alpha (6)