FWOF: I posted something similar recently with padlocks being removed from our bag several times.
No one seems to give adequate thought to the potential (as you raise) for apparently secure baggage having something added without your knowledge and then being discovered. There is a lot of time that baggage is outside of your care before being loaded, yet airlines don't seem to have any way of ensuring that your SECURE bags remain so after you check them in. Why can't they put a plastic cable tie round the entire case, so you have a visual clue when collecting at destination??
After all, Heathrow had notoriety as Thiefrow due to the amount of items extracted from passengers bags. Other airports may not be so prominent, but the opportunity is there at every airport. If the handlers have the freedom, skills and equipment to tamper with bags, then it is equally possible for them to add something. They do not necessarily have to believe in a cause, or money, they could be susceptible to blackmail that their thieving would be revealed.