Read the Heritage Foundation discussion of that clause. An indictment is not necessary. Only future crimes cannot be pardoned. Otherwise, it is pretty much unlimited.
As Alexander Hamilton argued in The Federalist No. 74, "in seasons of insurrection or rebellion there are often critical moments when a well-timed offer of pardon to the insurgents or rebels may restore the tranquility of the commonwealthThis is also cited in Murphy v. Ford, followed by:
Few would today deny that the period from the break-in at the Watergate in June 1972, until the resignation of President Nixon in August 1974, was a "season of insurrection or rebellion" by many actually in the Government. Since the end of 1970, various top officials of the Nixon Administration at times during this period deliberately and flagrantly violated the civil liberties of individual citizens and engaged in criminal violations of the campaign laws in order to preserve and expand their own and Nixon's personal power beyond constitutional limitations. When many illegal activities were threatened with exposure, some Nixon Administration officials formed and executed a criminal conspiracy to obstruct justice. Evidence now available suggests a strong probability that the Nixon Administration was conducting a covert assault on American liberty and an insurrection and rebellion against constitutional government itself, an insurrection and rebellion which might have succeeded but for timely intervention by a courageous free press, an enlightened Congress, and a diligent Judiciary dedicated to preserving the rule of law.Are we now in a "season of insurrection or rebellion"? Is Obama a part of that "season of insurrection or rebellion"?