User blog comment:Arbington/The Next Cycle/@comment-5337704-20120803074623

If the rachni queen is to be believed, she stated that the rachni were indoctrinated during the rachni war, which means the vanguard was awake prior to being discovered by humans. I have no doubt he may be on a timer, but he may also intercept certain signals that could awake him such as mass relay traffic etc. It is stated in game that he does monitor to see if the races are ready, to know he does it while he is dorment wasn't really mentioned.