you can access the police hq from the dwarven caves by taking a door down some stairs right next to the drill tank x and you can access the raccetrack by taking a door on a building in the cloud zone
you can also go back to the catacombs by taking the door behind you when you enter the racetrack but i don't know if that counts (i guess goblet gulag can be added next to goblet corp too huhu)
also SPOILERS : there's a new bunch of small zones (7 or 8 i think) following up the salt desert, ultimately leading to the warm black sludge (i'm sorry if my english is wrong)