Could it be the webhook which was used for Google Assistant integration before using it with Nabu Casa?
In my case, before using Nabucasa I set up Google Assistant integration by using the manual method: creating an app in the action console etc… Then I switched to Nabucasa.
Maybe the previously used Google Assistant integration stayed registered?
Just an idea 
I think that’s unlikely - in my case I’d never setup Google Assistant integration until enabling Nabu Casa.
Poking around, I think this is for a feature that is under development. GitHub - NabuCasa/home-assistant-google-assistant-local-sdk: Google Assistant Local SDK implementation for Home Assistant maybe?
Maybe I am not reading that page right, but it looks like it hasn’t been touched in years.