I can't say I was surprised by the opening scene, but from the questions asked I was certainly expecting something jarring to happen so I'm sure that's why. However, knowing the film is film noir and having the first few shots be so tranquil definitely put me on edge and made me feel like something bad had to be on the horizon. It is film noir, after all.
The mystery of the opening scene helps contribute to the film noir style, because aside from the sign telling us we're in Singapore, the audience knows nothing. There's no introduction of characters, setting, plot background, etc. Instead