This is not the first time that it has happened. BA has a very intensive inspection program in place to check for delamination of the internal honeycombe structure from the external skin. A number of years ago BA requested BAe to build both Upper and lower rudders so as to reduce the risk of this kind of failure.
The first time this kind of failure happened was on the round the world trip (I think). They landed in Australia (sydney) and ATC informed them that they were missing a section of there rudder. The crew were unaware until informed.