Deepening the mystery over the theft of a valuable painting by the 19th-century French artist Edgar Degas, the police in Marseille on Friday released a night watchman who had been detained for questioning the day before. He was on duty when the work, valued at about $1.15 million, vanished from the Cantini Museum there. The piece, “The Chorus,” a small pastel that shows singers performing on a theater stage, was missing when the museum opened on Thursday, and there was no sign of a break-in. It was one of about 20 Degas works on loan from the Musée d’Orsay in Paris for an exhibition of paintings with theatrical themes. It was to close on Sunday and travel this year to Italy and Canada.