Dec 17, 2001 An American Airlines flight was delayed more than three hours before takeoff at San Diego yesterday after a fake grenade used to test security screening fell out of a carry-on bag and rolled down the aisle, an airport spokeswoman said.The pilot of Flight 788 stopped the MD-80, bound for Chicago, on the taxiway and authorities removed the grenade. Before the plane's delayed departure, all passengers were taken off and re-screened.A woman was in custody and authorities were trying to determine how she ended up with a bag belonging to a security screener that contained the phony grenade.The woman may have inadvertently picked up the bag at the security checkpoint. After boarding the plane, the woman pulled a jacket out of the bag and the fake grenade rolled out. Leo/ORD