I have heard it said that no one can keep alert 24 hours a day, and that's what it means (to me anyway) when we roll initiative. Everyone concentrates on the matter at hand (usually combat) and acts in as effective manner as possible to resolve the situation (again usually trough combat).
The city guards in this example had been ordered by their commander to "keep alert", "double the patrols" (hence such large patrols) and "to arrest any suspicious looking people". They had probably been doing just this for several hours of the night already, and had had nasty incidents before this one. So, in my mind they would be open to the possibility of an attack from their target, and watching out for it.
On a regular night the patrol would probably have been surprised by the burst of violence from this guy walking around in the night.