In Episode 8, around 7:00...
Office complex, Freeman climbs up the tiny ladder up to the ceiling where some headcrabs crawl around:
"I guess they must've teleported in, but how did they know where to go?
Oh, maybe they don't. Maybe they're teleporting into the walls.
That can be why the building's falling apart. We're turning into the swiss cheese of the damned."
Shit, this series is so goddamn hilarious.