In The Case of the Phone Box Murder, the Blue Goose cafe was a eatery in an insalubrious part of Vienna which served as a rendezvous for criminals who wanted to engage an air transport service which would take them out of Austria to other countries in Europe such as Britain.
The air transport service was arranged by code. Customers interested in the service would ask for "ukranean borsch" and then present a "ticket" by tearing off a corner of the day's menu card with the word "ukranean borsch" on it. The cafe "served" Ukranean borsch on Wednesdays and Saturdays. The cafe was also equipped with a bolt hole which led to a separate exit on the Mariahilf Strasse. Customers could use this passageway to evade surveillance for the price of 100 schillings.
When Hans Muller was murdered in London, a fragment of the menu card was found at the crime scene. This led Biggles to surmise that he had been at the cafe the night before and that, therefore, he might have been traveled to London from Vienna the night before. Biggles and co. proceeded to Vienna to find the cafe. After three days of searching, Ginger located the eatery. Biggles then visited the "Blue Goose" pretending he was interested in an air passage to London. This enabled him to penetrate the transport network. At the end of the story, the Blue Goose was raided by Austrian police. Among those arrested were five deserters from the American military forces in Europe.
The Blue Goose is a fictional location but the Mariahilf Strasse, more properly Mariahilfer Strasse or Mariahilferstraße, is Vienna's main shopping street.