Given that this has happened twice to the OP, there HAVE to be other instances of it happening to other customers.
Because it is basically not possible for it to have happened only twice and for both of those two instances to have happened to the same person.
Therefore, assuming that this has happened to others as well, it should be fairly straightforward for Dell to track down what is common amongst the data for the stolen items (they probably have a lot of information) and therefore to find the guilty parties (or maybe I just read too many mysteries).
At any rate, as soon as they knew that it had happened to other people, they should have immediately made good on the camera (sent another one) that, of course, they would have checked thoroughly prior to sending.
If they did not find any other instances of this happening, of course, then they would need to be investigating the OP (which is another possible reason for their delay in sending a third camera).
The opening gear photos prove nothing. Even an opening boxes video would prove nothing. It would be incredibly easy to carefully open the box and then reseal it so that it did not look like it had been opened. When it comes right down to it, I have to give Dell credit (even though I would never order from them--I stick with B&H, Adorama, and newegg) for sending that second camera.