Since this article was written there have been several doubts raised as to whether the signature in the visitors' book at San Lazzaro is actually that of Marcel Proust. It has been suggested that the signature could be that of the Swiss writer Marcel Rouff (1877-1936).
The most convincing discussion is in this German article by Reinhard Pabst:
If the signature is not Proust then there is no other available evidence that Proust ever made a second visit to Venice in October 1900.