The original ending involved Faye meeting Cop #633 at the airport and the two characters flying off to another country. However, when Wong couldn't agree terms for his ideal location in an airport, he decided to opt for an ambiguous ending.
Faye Wong is better known in Hong Kong as a pop singer. Indeed, that is her singing on the cover version of The Cranberries song "Dreams" that we hear during the film.
The woman in the blonde wig had no apparent opportunity to get the beeper number except from Cop 223 himself the night before, and he wouldn't have started to abandon his beeper as useless if he had just given someone new the number.
He Zhiwu, Cop 223: At the high point of our intimacy, we were just 0.01cm from each other. I knew nothing about her. Six hours later, she fell in love with another man.
Cop663: Did I leave the tap running, or is the apartment getting more tearful? I always thought it would cope okay. Didn't expect it to cry so much. When people cry, they can dry their eyes with tissues. But when an apartment cries, it takes a lot to mop it up.