Don't know where you got your info. Cardinal Law stayed in Boston while the Attorney General did his investigation. After the AG had his press conference stating that the State wasn't going to charge the Cardinal with any crimes, Law left Boston, and went to Maryland, where he stayed in a monastery for a year, in prayer.
After THAT, he moved to Rome to live at the Basilica of St. Mary Major, which was his parish in Rome (I believe every active Cardinal has a 'home parish' in Rome). Now that Law has reached retirement age, and is no longer an Active Cardinal, he is now Cardinal Emeritus of St. Mary Major, and there is an active Cardinal in his place.
Information came from the horse’s mouth.