What happened to Rahim Kahn? Why does Amir leave the hospital in Peshawar and go to Islamabad?

 Rahim Khan had left Peshawar, leaving behind a key and a letter for Amir. Amir left the hospital against Dr. Faruq's advice. He planned to use the money from the safety deposit box to pay his bills, leave Sohrab with the Caldwells, and fly home. When Farid arrived with Sohrab, he explained that there were no Caldwells in Peshawar, nor had there ever been. Farid drove Amir and Sohrab to Islamabad