Hi there,
I am new to this forum, but I am using kodi for quite a longer time now. Right now I am running Kodi 18 beta builds on my Raspberry Pi 3, but the problem, which I now will describe exists on non-beta builds of either OSMC or LibreELEC as well:
Before I switched to kodi, I used my old Raspberry Pi 1 with runeaudio to playback audio via mpd and the analog audio output. My AVR has an auto standby function, which is set to 15 minutes, so each time there is no audio playback for longer than 15 minutes, my AVR turned off into standby. So far, so good.
Now since I switched to whatever build using some version of kodi and the hdmi audio out, my AVR ever stays turned on, no matter if I don’t play anything for hours. I switched the screensaver to black screen, turned of GUI sounds, turned off silent audio signal and tried a lot of other things but never have had success in fixing this.
So I want to know, if this is just me and my AVR (Denon AVR X1000) or if most / everybody of you using a similiar setup (Raspberry Pi HDMI Out → connected to AVR HDMI In → connected to TV) are experiencing the same behaviour?
I find this quite annoying because I often like to listen to music late at night and if I fall asleep my AVR will stay connected until I remind to power it off by hand the next day and it is consuming a lot of power for nothing.
If there exists a way to check via terminal if there REALLY is audio playing via HDMI, I even would consider scripting a little bash script which will run as daemon and power off my AVR using HTTP requests. But I don’t know how to check this, because kodi.bin will be running 24/7.
I hope you have been able to understand me, because I am not a native speaker 
Hope someone can help me with this 