Right click on the character to open the properties, under AI scripts, click on "main". Find the corresponding script for that character. (Should be in the Zombie Apocolypse folder, if your still using those characters.) Then open up the one you want to edit (you can use notepad, or download FPIEdit pad) and you will see a line like this near the top.
;DETECT PLAYER
:state=0,plrdistwithin=1000,plrcanbeseen
Just change the value to whatever you like.
I recently learned to sleep with my eyes open. ..... God am I tired