here's my theory:
after the closure of the superportal in ep2 the combine units remaining on earth were wiped out. after many generations mankind forgot, and all that's left were remains of the citadel and some recordings from gordon's HEV suit. Eventually mankind made the dark energy construct, the reverse-engineered citadel (which they essentially worshipped for speeding up their technological progress), and the decacos system (thats what the sound files call the map's announcer). decacos is fully sentient and capable of feeling pain. eventually it dies out in the reactor meltdown