"Hostile alien actions" when no one is in their room
You do perform all actions listed if possible. So in the case where there are no players present steps 1-3 don't happen and if there are no unlocked systems present step 5 doesn't happen. This is poorly worded in that section of the rules but if you look closer at the example gameplay they give it shows step 4 happening even with no players present.

Rules


  You’ve used up your actions, so it is time to draw your
  end of turn Alert. You draw “Patrol.” Two Alien Patrol
  appear in the Warden’s Office Hallway. Luckily, no one
  is there, but these are Patrol Aliens. They don’t just sit
  still and wait for the action to come to them. When you
  PERFORM HOSTILE ALIEN ACTIONS, they both move
  into the Mess Hall and find you there. An alarm sounds
  and the Concealment Level goes down by 10%. One
  of them injures you and escorts you to the Infirmary/
  Holding Cell while the other stays behind, ready to
  continue its patrol on subsequent turns.


(added bolding)

If follow through the example it shows two patrol aliens in a room with no players and has them moving to the next room where they find a player. Once they find the player they perform steps 1-3 as normal but they do not perform step 4 again. 

The critical thing to note that unless players are in sequential rooms along the patrol path having them not move when no player is present really takes away alot of the power of the patrol aliens and makes it a lot easier to avoid them.