Maybe BT scratched it when he cleaned the sensor prior to sending it in. That said, Nikon scratched my D600 sensor during one of the times the shutter was replaced. They claimed I scratched it--showed them the very first shot I took after getting the camera back.
In any case, I'm glad they replaced yours while they had the camera so you didn't have to send it in again.
Very unlikely that air from a rocket blower would do that.