I think there is a difference between hearing it in the ducts and just knowing it is in ducts. When you can hear it in the ducts near you there are higher chances he will appear if you dont walk slowly. AT least this is the impression I gotIf alien is in the ducts, you can walk normally. If he rotates between floor and ducts, then the time window for walking is very small.
Actually I think CA would make an excellent System Shock 2 reboot or even a great System Shock 3.I hope that next game from CA