Since you are they one opening the window with a /window command, you (or your script) will always know when/why a window was opened and what it's for. Therefore, if you wanted a more robust on OPEN to detect when even YOU opened a window, you can either use signals to check for on OPEN or you can write your own /window-type alias and have everywhere in your script call that, including all of your on OPEN events. In the on OPEN event, $event is "open", which your alias could test for and handle accordingly.
