This is a fairly common story which is told in many variations. I did not hear the version involving the emperor and patriarch, but rather that it was one of the cathedral priests celebrating a regular daily liturgy and when th Turks burst in took the gifts and walked through the wall of the altar. It is said that if someone were to begin the liturgy without the gifts, at the precise moment he disappeared, the priest will return through the wall and finish the liturgy that was interrupted. I don't doubt that there will be a multitude of other variations on the theme of the disappearing priest and the unfinished liturgy at Hagia Sophia. Fr David Moser