I was out shooting today and my D810 developed this problem. Even when you pop the battery out and leave it out for a while and put it back in it's still on. Strangely, I managed to turn it off a couple of times by changing the setting on the left top dial (the one for shooting method). I was shooting with mirror lock up, and decided to switch back to single shot, and the camera turned off then.
Also, the image review decided to stay on permanently until I half pressed the shutter.
Any ideas what might be going on? I guess I might try a factory reset if that is possible. It's still under warranty, so I can get it fixed, but I'd prefer not to have to send my camera off for a month if I can fix it myself.