SOMA Enemies (major spoilers)

[cantremember]

It's the robot girl, although she doesn't "run downstairs and chase you". She's already standing in the room before the corridor once you get there, and starts chasing you once you're in the corridor.
She also kind of just appears there, as you can meet her in the power room, and then go downstairs and meet her there again without a logical way of her getting there.