:smile:In all actuality, the real question we should be asking is not where the Chief is but when he is.
Most estimates put him at 536 years before the first firing of the Array and before the Flood were uncovered. The Chief is revealed to be Didact and Cortana is actually the Librarian, given a Forerunner body upon accessing their world. The two's romance was cut short by the Flood and 300 years of bloodshed.
Does this come from the GS 343 line, "Last time you asked me if it were my choice [to fire the ring]..."?