So if I have a .weapon (not an item_collection) placed on a map. If I then pick it up, is it considered destroyed if I were to use a script like:

if this weapon is destroyed, then execute some commands

no, it still exists. you still refer to it by its object name. Anyways, you can only check unit's life anyway, there isnt even a script to check if a weapon is dead (until halo 2)

Then could I use a trigger volume to say,

if this weapon passes through this trigger volume, execute these commands

?

should be fine.

thats just for any weapon of a certain tag location, following for his previous stuff, hes checking if a named weapon goes through. Anyways, there is no reason why a child shouldn't be found by a volume_test (when you carry weapons, they are childs. when your in a vehicle, your child to the vehicle, etc)