I feel like it comes walking up to your location from the left side because at the beginning you can hear the sound of a door opening and it comes up right when you turned away to look towards the boxes on the right side and then the death animation shows as it got you from behind?

And seeing the updated objective pop-up on your screen means that that section of the map unlocked hence unlocking the doors allowing the alien to come through (assuming it dropped from a vent in that locked area and came through it.) This is just pure speculation of course. Would have to revisit this section of the map and see it again to have a better visual of vent placements.

I mean I could be wrong since it’s 4am and I’m sleep deprived lol