There have been credible reports of Mitrice Richardson sightings in Las Vegas and the L.A. County sheriff's department plans to hold a news conference there tomorrow. "It's not definitive, it's not absolute," said department spokesman Steve Whitmore. But it's enough to make appeals in the Nevada city, he said. Richardson has been missing since being released from custody by the sheriff's station in Agoura Hills last Sept. 17. She had been taken in following an incident at Geoffrey's restaurant in Malibu.