Would need to be a very skilled mechanic to a) see in a closed tube b) see under paint c) see past the facing off the crown.
Not true! If a small shunt, as described, was sufficient to dislodge the steertube, there is no way that there was not also some recognizable play and auditory evidence beforehand. When you assemble a brand new frame, you should face the fork and ensure that the headset fork crown race is fully seated. This action will apply more pressure that should occur from a small shunt. Then once the fork crown race is applied to the fork and the cups applied to a faced frame you need to stress the assembly. Again applying teh same force that can occur in a mild shunt.