In Die Hard, Al Powell is told to check out Nakatomi Plaza, however, he wasn't told about someone calling in about the building take over or gun fire (McClane).
When he walks over from his car, we can see flashing lights on top the building, are those lights the gun fight between McClane and Karl? If so, why didn't he question the receptionist when he walked into the building?
Given the distance he probably didn't hear the gun fire, but the receptionist (who is really one of Han's men) won't know what's going on, and be caught off guard. Al could have asked if some type of construction was happening and if the receptionist had said yes, ask him what was being done - anyone working there for a long time would at least know what was being done on top of the building. If he said he didn't know about any lights, then Al could have investigated further.