OK - a few more information just in case someone cares 
There is a channel “idle” for which you can create a switch, unfortunately this channel does not always switch back to “idle” when the Google Home is not playing anything.
Switch Google_Home_Bad_Idle "GH Bad Idle" <network> (gAssistenten) { channel="chromecast:audio:Bad:idling" }
But you can use it when you startup (switch on power) the Google Home and try to launch a stream.
In my case I do wait for like 20 seconds and then try to lauch a stream, and check the “idle” state, if it is still “idle” I do wait for 1.5 seconds and try again. This seems to be the trick.
As I said before unfortunately this channel is not always switching back to “idle” when you stop your stream either per voice or per Player control item (sending the PAUSE state).
So when you re-enter the room and the Google Home is still on Power you have to check the Player control instead of the “idle” item.
All in all this now works pretty stable.
I will write the solution in a seperate post as tutorial as I believe this may be usefull for others as well.
Here is the link to the tutorial: Chromecast / Google Home - automatic start playing a stream when motion is detected in a room