I read an interesting article recently (forget where) about security checks and X-ray machines. It seems that due to the mind-numbing nature of the job, a huge number of mistakes are made -- no surprises there -- and that's before you start to think about guns designed to have the X-ray profile of a bunch of keys... I'm not surprised that a closed Swiss Army knife gets through.
To sort this out, the next generation of machines can superimpose images of known bad objects onto the shadows of pax baggage on the screen. The operator then presses the "Is it real?" button and decides whether or not to pull the bag for a search. If the operator misses the false object, then this is recorded and if too high an error rate is observed for a particular operator action can be taken.
Which should make things better, although I doubt that the range of knitting impedimenta allowed in the cabin will increase significantly...
R