Achive audio from early season episodes is heard as Carter walks the halls of County General one last time, including Dr. Greene telling him "You set the tone, Carter."
When Greene sat in his chemo session, he glances over to the peaceful paintings to his left. The second one is of a sunrise, which presages his later death. In the message sent back to the ER by Corday after his death in Hawaii, she says that "Mark died at sunrise; his favorite time of the day."