The department store visited by Alice MacDonald and Bob Matthews was a genuine shop, Barkers of Kensington. The 135-year-old art-deco establishment closed in January 2006.
The balcony scenes that look over the river Thames are from the Savoy Hotel in 1956. The Thames was declared dead (extremely polluted) the following year, 1957. It has taken many years to clean up but now the once brown smelly river can support life and is home to several species of fish.
The curious title (a clear allusion to Sherlock Holmes) is only referenced once, when Phil helps a lost, bespectacled man find his way in the fog.
The film maximised 20th Century-Fox's "frozen funds" - money earned in England by the studio which, by law, could only be spent there. Hence, the extensive location footage is genuinely shot in London, whilst the interiors are all Los Angeles-based.
The scars on Van Johnson's face in this film are real, though make-up seems to minimize them. While filming Kampf in den Wolken (1943), Johnson was in an automobile accident and thrown through the car's windshield. The plastic surgery of the day could not totally remove his scars.