Oh, that's a combination of some env_renders and this:
http://twhl.info/tutorial.php?id=64After you set this all up, you must make a brush-based key, similar to the one picked up by the player at the keyhole, then tie it to a passable func_door_rotating with Render Mode "Texture" and FX Amount = 0 and set up two env_renders - the first one with Render Mode "Normal" and the second one with Render Mode "Texture" with FX Amount = 0.
When the player activates the trigger_once that checks if the multisource is active, multi_manager gets activated and activates the first env_render (which makes the key (func_door_rotating) visible), then the key (func_door_rotating) itself, and then activates the second env_render which makes the key invisible again.
There must be an example map somewhere at the vault. If you want you can try to find it. Good luck!