I think it was mostly due to lore reasons that the alien doesn't use vents much. Axel introduces the player to the vents as means to survive and it seems they tried to stick with that. In the DLCs and on Nightmare difficulty it is more free to use the vents, though once on Normal in the mission where you need to replace a cylinder for the elevator I went underneath the group of survivors through that big crawlspace with a flare and when I was almost at the exit I heard steps behind me that were not my own...
Every time I got a little too cocky - like running around waiting for the train after Axel's death - the game had a surprise for me, telling me that it *could* make me reload the last save, but it doesn't have to as long as I behave and hide behind a crate.
NerdIII
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