TonyRanger -
0 means the unit doesn't know anything about the other unit
4 means they know the exact position of the unit
From messing around with this, it seems that ANY death or explosion raises the level to 1.4 or 1.5 automatically, even if they haven't seen you.
I.e. - If I sneak in and plant a satchel charge next to a tank then sneak back out and detonate it while completely out of sight - anyone near the explosion has their knowsAbout level (against me) go instantly to 1.5 - even though no one has actually seen me at all.
This also fires the Trigger "Detected", even though no one has really detected you...they are just aware that someone must be around because something blew up.
Same happens if you snipe at someone with a surpressed rifle - anyone around the guy that was killed goes to 1.5, but they don't actually have a clue where you are.
To make a realistic detection script I'm guessing the threshold needs to be around 2...in my case I make all the guards doMove to my last spotted pos when anyone has a knowsAbout higher than 1.7.
I use a "Detected" trigger to kick off the script, and the script then polls every 40 seconds finding the highest knowsAbout level (of every bad guy on the map). Once it reaches 1.8 all hell breaks loose and they all head out to the last spotted pos.
After that it polls every 4 minutes, in case they loose contact with me and then subsequently re-spot me (or anyone in my team).
I'm surprised it is this complicated to script though - some more events would be extremely helpful - like an event for Behaviour changes, or an "Engaged" event indicating an individual is now under fire and/or firing at hostiles. Would make mission scripting a lot simpler!