I have placed backpack at position where it is possible to spot various smart terrains at once within line of sight(more-less.)
Placed backpack on reference spot, moved around map and used find object tool in debuger to give distance from backpack. Taked notes about distances and placed again additional backpacks with distances in stash descriptions. Not 100% accurate, but close enough to give you idea how far NPC can spot you.
All these hands look hilarious :)
Well, I'm not so good when comes to editing pictures. I just made several screenshots and copied parts of pictures from different screenshots and merged everything up on one picture :)
But, served purpose well as reference points for further AI tests.