The surviving Forerunners (the IsoDidact, Chant to Green and an unknown number of Lifeworkers and Warrior-Servants), after reseeding the galaxy of life, left the Milky Way to go on the Great Journey. It's implied that they went to track down the dormant Precursors, as the IsoDidact mentions them in Halo 3's Terminals, saying "we can follow in Their footsteps". We know they're still alive in some form as of 2559, as Chakas has discovered the location of Chant to Green (who he thinks is the Librarian because she passed her title of Lifeshaper down before stranding herself on Earth). On a number of occasions in their 10 million year history, the Forerunners have travelled outside of the Milky Way. The ancient Warrior-Servants who fought the Precursors left the Milky Way to pursue their enemy to Path Kethona with hundreds of thousands of ships, this is discovered by the Librarian and the crew of the Audacity who likewise leave the galaxy to go to Path Kethona towards the end of the Forerunner-Flood war. Both the Greater and Lesser Arks were constructed outside of the Milky Way.