Isn't there an event for someone finding an item? Couldn't you check if it's one of the halloween masks? Although I'm not aware if you can tell the difference between a present and a trade.
Looks like item_found does all those things
http://wiki.alliedmods.net/Team_Fort...nts#item_found
I know that the item isn't found right when the present is picked up, but it's gonna be the closest you'll get, so it's a good place to start.