Scenario 3 is more likely. Richard Reid traveled around the Middle East with his shoes in dry runs to see if they could be detected, before his handlers put in the fuses and put him on a plane headed for the U.S.
It can't be too hard to choose a "standard" location on all aircraft to hide blades which will not be noticed, but which can be found by future terrorists. Under the carpeting in the johns, behind fixtures, joints in trim, etc. Having one guy with an otherwise clean record make the deposits reduces the chances of discovery. On a trip with numerous connecting flights, blades can be dropped onto several aircraft and the frugal terrorist can even save money by buying tickets with numerous connections.
I hope they did check the other passengers to see if he had buddies.