In the process of prosecution, it's sometimes not desirable to reveal an ongoing indictment to the public - especially to the person being prosecuted - mainly for safety reasons. This may include the possibility of the suspect fleeing upon discovering they're being prosecuted, as well as becoming violent against any possible witnesses involved in the case. In these cases, a secret indictment is used - this means that the indictment is kept concealed until the moment of the arrest of the suspect(s), at which point their charges are revealed to them fully.
Secret indictments have various other names, such as "silent" or "sealed" indictments. Some have questioned their legitimacy in regards to a person's privacy rights, though the general consensus is that secret indictments are absolutely legal and do not violate any privacy rights. It should be noted that there have been cases in which authorities have attempted to retain the nature of a case secret even after the arrest of the suspect, which is actually a violation of human rights, though these cases have been dismissed as well after proper investigations had been conducted.
Secret indictments are popularly used against major figures in the criminal world, and the investigations attached to them may last for years before any arrest are made. It's not rare for secret indictment cases to end without any charges being pressed at all, which depends on the authorities' ability to build a solid case.