Having just finished learning about Triple A
I can tell you that pre the Lockerbie accident it was the PIC's decision weather a (fail to board) FTB passenger's bags were off loaded, a passenger failed to board the Pan Am 747 to JFK and 10 mins out of LHR the plane blew up.
After the Lockerbie accident the rule was overridden and as of now any passenger that FTB will have their bags removed from the aircraft, no if's buts or maybe's
Tripple A means that the number of bags in the aircraft must match the number of bags checked in, if there is more you have to find it and remove it, if there is less that doesnt matter the plane can still go, if there is a porblem, some one in baggage bay took off to many Bingo stickers, then the bags are off loaded onto the tarmac and every one is called down to identify their bag.
At perth INTL there are 4 levels of checked bag screening (CBS) if a bag fails the 1st screen it is moved onto the 2nd screen point. if a bag passes the 1st screen it is sent to the bag belt for loading onto the aircraft, if a bag does not pass the 4th test it is loaded into a bomb trailer and taken to a remote loacation of the airport to be further investigated.