Not defending the cop necessarily, but as a firefighter we respond to a lot of calls where the police are responding as well. We do have certain calls that require a "silent" approach, such as domestic calls, suicide attempts, or any time there may be an emergency that might be aggravated by the sudden noise and activity of lots of cars and trucks arriving with all the "bells and whistles".
Police almost always use the "silent" approach on burglary calls or suspicious people lurking about.
It MAY be that the cop you saw had used his light/sirens enroute to your neighborhood and shut them off when he got there but still has to hurry to the call.