Because of the part about receiving a silver camera instead of a black one, which means somebody at Amazon had to open it. Perhaps they bought the camera in with them, and it would have been noted before in the morning, thus leaving with a camera wouldn't have been unusual.
It might not be the case, but as I said before, somebody at Amazon looked at the camera, not really sure how you could not see that it was incorrect.
Surely it would have been easier/safer for the employee to put something of equal weight like a rock in rather than a camera? Why go to all that effort to bring in a camera if you know it's going to be clearly noticed it's the wrong one? Did the camera fit in the tailored/fitted internal packaging and did it weigh the same as the other model? Didn't your dad notice it had been opened when he was wrapping it and then have a look inside? Story sounds fishy to me.