Kill Xeno | Mission 1: Distress A: CM Guide

You can use the motion tracker to locate the fugitive Xenomorph, but it's not a requirement. If so, you'd have to be ready to quickly select the firearm from the inventory. The alien should be hiding near the stairs you took to enter the quarantine zone, so backtrack to the rooms you've already checked out. When you locate the Xenomorph, use any weapon (preferably the pulse rifle or a shotgun) to get rid of it.