Please see image below - My thought was to blur the noise - and / or combine it with different renders using a different seed value.
My question is - What combination of render passes will give me the image showing up as the difference? Or, is the difference the only way to get it?
Your setup is fine. My way to go is the make 2 renders: without volume / with volume and all the objects in a black material (holdout or nothing). I prefer to separate because I can set different settings for each layers, remove or add objects casting volume shadow, etc.
Thanks for the reply VincentG - I just thought there would be a way to pull the information directly. World settings - volume scatter material and the effects would seem to be passed from the environment pass.