You can't find it because there's no such script in the standard game template. But it can be easily added. Locate the "interface\inventory.def" file and add a SCRIPT line to the inventory window definition, like this:
INVENTORY_BOX
{
ITEM_WIDTH = 65
ITEM_HEIGHT = 65
...
WINDOW
{
X = 0
Y = 0
WIDTH = 800
; ===== ADD THIS LINE =====
SCRIPT = "path\something.script"
...
There you have a script attached to the inventory window (of course, you need to create that new script file as well).