Many reasons why drugs don't get detected in transit, not least depending on the length of transit. Likewise scanning on departure prior to check in is a great way to bring the whole system to it's knees, this used to happen years ago prior to check in looking for explosives, with todays traffic volume it would be a nightmare. Scanning after check in ? So if you identify a bag you then have to identify it's owner and make sure they don't board the flight. Try doing that at an airport like Swampy. Won't be long before one of these muppets gets caught at Singapore, mandatory death sentence for 500g of cannabis, they executed a guy this week for 300g of methamphetamine. Maybe publicity like that might make even the dimmest think twice.