From the Joystiq article,
" An unseen network of air ducts connects every room and corridor, so any alien that slips out of sight will eventually emerge elsewhere. Probably behind you. This network of access pathways creates some interesting artificial intelligence challenges for the dev team, as Gearbox veteran and Colonial Marines senior producer Brian Burleson tells us. "The Xenos don't necessarily spawn from the same place every single time. We have this concept called 'bloodstains,' and that means if Xenos die in a certain area, they'll stop coming in that way."
This makes me very, VERY, happy.