Although The Third Man is (famously) set in Vienna, and makes use of numerous locations around that city, much of the studio work and other shots were filmed in London.
This would apparently include at least some of the back projection footage made for the driving scenes. During the drives to and from the hospital near the end of the film, several London buses are visible on the road. (These wouldn’t be the famous Routemasters, though, as those weren’t introduced until the 1950s. They’re more likely to be an earlier model, known as the Regent.)
These kinds of things are often easier to make out in moving footage, but above you can clearly see the outline of a bus behind the brim of Martens’ hat, and on the right of the picture below, even the bus route is tantalisingly close to being legible.