[BUG] Scene 'scenes\episode_1\c17\al_tunnel_01.vcd' missing!

Any HL2 campaign or episodic maps cannot be played because events don’t trigger. Alyx won’t ever move, combine won’t ever break down doors, etc. It’s as if NPC thinking is off when it’s not. Whenever an event is supposed to trigger, a message such as the one in the title it printed.

Have you tried re-mounting HL2?

Yes.