Section 15. The grand jury shall, during its session, make daily return to the court of all cases wherein it has finally determined not to present an indictment against an accused person held in custody pending its action, and such person shall thereupon forthwith be discharged by order of the court unless he is held on other process. Whoever is held in custody on a charge of crime shall be discharged if he is not indicted before the end of the second sitting of the court at which he is held to answer, unless the court finds that the witnesses for the prosecution have been enticed or kept away, or are detained and prevented from attending the court by illness or accident, and except as provided in the following section.