There must be some randomization involved there, because, for me, there were plenty of sections, even late in the game, where I didn't see the alien for extended periods of time.
There are actually two AIs at play - the AI of the alien itself that dictates how it will behave and where it will look - the alien is also capable of learning the player behavior and adapting accordingly (if you use the flamethrower too much it stops being afraid of it, if you hide under desks/in lockers/etc. it will eventually start looking there for you) - and the director AI, whose primary job is to tell the alien when it needs to come out to ratchet up tension and when it needs to go away to give the player some breathing room.
My main issue is that I mostly avoided the alien coming out of the airducts, outside of the scripted sequences - but you can always hear the alien near you making noise in the airducts. It is constantly near you, waiting for the player to make noise so it can start hunting.
That was the annoying part - at no point does the alien fuck off and bother someone else, he's always tethered to the player and in the vicinity.
There's actually a mod that extends the "leash" and lets the alien roam much farther away from the player on the map, and I found the experience much more enjoyable than having to hear the alien crawling in the shaft overhead just waiting for me to make a sound.