Btw. if you haven't, I very much recommend watching a vid on how the Alien AI was programmed. It's quite fascinating and one of a kind insanity the devs pulled off.
In a shortcut - the Alien has got two AI "brains", basically.
One brain is a "cheater" - it knows exactly where you are. It's done for the gameplay reasons, simply so the Alien doesn't wander around on the opposite side of the station just to never find you. It also knows when you complete a certain mission goals and whatnot. But the trick is, this brain cannot control the body of the Alien. It can only "advise" the other brain on your location once in a while (and it cannot do anything if you're actually close).
The second brain controls Alien's body, but it doesn't know where you are. But it can "hear" (i.e. gets your location if you make a noise) and "smell" (i.e. it knows where you WERE a moment ago, for example if it walks through a hallway you've been to a minute ago, it can catch your scent and tries to track you down).
That's why the Alien is so life-like and very unpredictable in what it will do. It doesn't have any "patrol patterns" so to speak and no default attack modes. It really does what it wants and what it feels is the best course of action against you. The only thing hard-coded into its brains is that it wants to kill you.
It's truly fascinating stuff. (for me, anyway.

) I've even seen devs playing it and being amazed (and often quite surprised).