Richmond police are trying to locate a missing 21-year-old man who witnessed the October rape of a 16-year-old girl outside Richmond High School’s homecoming dance, a police sergeant said today.Salvador Rodriguez was reported missing Dec. 16 by his family, who had not seen him for 10 days, Richmond police Sgt. Bisa French said. Rodriguez was one of six people arrested following the Oct. 24 incident, in which police believe several people robbed, beat and sexually assaulted the victim, a Richmond High School student, in front of as many as a dozen onlookers.Rodriguez was released from jail on Oct.30 after prosecutors concluded there was insufficient evidence to charge him.French said today that after his release, Rodriguez was “known to go from friend’s house to friend’s house” and that “he was concerned that people thought he was snitching. “Richmond police are continuing efforts to contact him. French said she does not know if his disappearance is connected to the rape case. “We don’t know where he might have gone,” she said.