Thank you for this, it’s really interesting.
You have probably read this http://wiki.phoenixpoint.com/Combat#Detection
Tbh, I was not happy with this chapter prior to reading your post and I’m less happy with it now 
Long, long time ago @pantolomin had a look at the code and shared some info on how detection worked:
Huge bug in how "hearing" is handled,
but my guess is that it has changed several times since then.
Rereading it now I see that regular hearing range is 5 tiles.