A few nights after he resigned his post as secretary of state two years ago, Colin L. Powell answered a ring at his front door. Standing outside was Prince Bandar, then Saudi Arabia’s ambassador to the United States, with a 1995 Jaguar. Mr. Powell’s wife, Alma, had once mentioned that she missed the ’95 Jag that she and her husband had traded in a while back. Prince Bandar had filed that information away, and presented the Powells that night with an identical, 10-year-old model. The Powells kept the car — a gift that the State Department said was legal — but recently traded it away.