Out of the corner of his eye Casey thought he saw a flash. A quick flash of light, when he turned his head though he saw nothing out of the ordinary. The soft thud in his head was still there and the eyes that had been glued to the computer screen most of the day were beginning to demand a rest. Stubbornness won out, however, and he pushed on.
Five minutes slipped by. Then fifteen. Maybe half an hour. As Casey looked at something on the edge of his screen, there it went. Another flash of light. Brighter. Definitely longer. This time from out in the hallway. Without thinking, Casey let go of his mouse and walked from behind the desk. Slowly. The pain in his head was gone, replaced by the pounding in his heart.