Law enforcement authorities are looking for what they believe is a hit-and-run driver who killed a mentally ill, homeless man in Milwaukee.  The body of 63-year-old Roderick Greengrass was found in an alley early Sunday morning.  Investigators say his injuries were consistent with being hit by a vehicle.  A Silver Alert had been issued for Greengrass after he went missing from Aurora West Allis Medical Center.  People living nearby had reported a man going through garbage in that alley and a black pickup was seen driving through the alley shortly before the body was found.  Greengrass suffered from delusions and had been staying at the hospital until his family could arrange a guardianship.