Only issue I have of that is: Dallas get to the position they saw the xeno on the motion tracker, as she says: "it should be there, you sure you dont see it?" Its just then they lose its signal.
When the signal comes back online, its at a different location, moving towards the same position again.
So: the alien was there dallas was, ran off at the exact time the signal gets lost, turns around and starts heading back to the exact same spot it was at earlier when the signal reappears. Sounds kinda too convenient.
As they say the easiest solution is likely the correct one: it was there lambert saw on the tracker, Lambert just misstok which level it was at, one level below where dallas came from. Then the signal gets lost, and then reappears at a different location, heading towards dallas, making them panic and dallas decide to not go back where he came but go down one level, right into the arms of the xeno who was sitting there the whole time. Im convinced it was Asch who made the signal to disappear, load at a different location and have it move towards dallas to trigger a mistake.
But thats how I see it, hard to tell, you can be just as easily right.
Nothing wrong with the air densisty, its the same system the motion trackers use in aliens from my understanding.