Successful detection of a crime

The detective opened the door to the Company director’s office and surveyed the scene.
director’s head lay on the desk in a pool of blood. On the floor to his right lay a gun. There were powder burns on his right temple indicating that he was shot at close range.
On his desk was a suicide note and in his right hand was the pen that had written it. The detective noted that death had occurred in the past hour. He also realized that it had not been a suicide but a clear case of murder.

How does the Detective know?