The address of The Parasol Corporation on the name board is 221b Baker Street, this is the fictional London address of Sherlock Holmes.
The movie was actually filmed using 2 different locations that were 'mirror' opposites. So about half of the footage had to be 'flipped' to match.
The character Hitomi was named after the directors favorite chapter of 'The Grudge'. Fuko was named after the character in 'I My Me! Strawberry Eggs'. The director used the name Naruimi because they wanted a character whose name started with an N.