In my case, I'm having a full power down of the printer after several hours.
My printer does as you describe after 15 minutes or so which I'll call "standby". But then after several hours in standby it will perform a full shutdown. I was in the room once when it happened after the printer was in stancby for about 5 hours. It made a bunch of churning noises much like those when the printer is initially turned on and then it fully shut down. No LCD, no power light and no communication. Given how long it takes I have not had chance to determine if the timing is fixed and consistent or random.