OK, I've studied Sancre Tor Prison in the Construction Set (CS), and I've figured out how this works...
You have to find and defeat the four Undead Blades in different parts of Sancre Tor. As you defeat each one and talk to him, he heads for Sancre Tor Hub. The one in Sancre Tor Prison is the Ghost of Valdemar. On his "journey" out of the Prison, he takes a shortcut as only a ghost can...
He walks right up to the very spot where the YouTube poster is facing the wall of rubble, silently fades out of existence on that side and just as silently fades back in again on the other side. He's "walked through the wall" and, through the (convenient) hole in the rubble, you see him reappear on the other side and walk away. But you're not a ghost, so you have to go the long way around. This lends a great spooky moment to the Quest, reinforcing your belief that he's a ghost who's no longer even aware of you, just determined to do his final duty.
OK, here's how the "trick" is done...
1. After talking to him, he switches to an AI Package called MQ09PrisonGhostDoor. The package type is Travel, which makes him walk to a destination waypoint, in this case an invisible marker beside the rubble called MQ09BlockedHallwayMarker. It's on the same side of the rubble in the YouTube video.
2. Once the package completes its job of getting him to the marker, his script's OnPackageDone block runs. This activates a hidden door object called MQ09NewGhostDoor01AREF. It's actually a visible trapdoor that's sunk JUST out of sight under the floor, beneath the low flat rock you can see in the video.
3. The door object teleports him to a door marker (arrival position) on the other side of the rubble called MQ09NewGhostDoor01BREF. Because the door object is activated by a script, not an NPC, there's NO door sound. Also, there's no visible door, but he does the usual fade-out on going out through a door and fade-in on coming in through one. Coupled with the fact that he's a ghost, this silent fade out/in creates the neat spooky effect of him walking "though" the rubble.
Note: The door is owned by the Undead Blades faction, hence the icon being red. The lock type is "Needs a Key" but the required Key is set to NONE. This is the way Bethesda have chosen to handle doors which they NEVER want opened, even if you use the ToggleClipping cheat, e.g. Merchant inventory chests, and hidden doors.
On PC, I can't get the lock icon to appear at all. But I have a theory as to why this is which also explains what's going on with the Xbox360 version...
To detect which object you're "aiming" at, 3D games fire a "ray" into the 3D scene (effectively "sighting" through the crosshair) and figure out the closest object the ray intersects with. Oblivion uses the same aiming technique. If the crosshair is "over" an activatable object which is within the Player's activation distance (reach), the game displays info ABOUT that object.
For performance reasons, the game doesn't do intersection tests on a high complexity object mesh, but on its associated low complexity collision mesh instead. The amount of 3D maths to determine an intersection is greatly reduced, and less maths calculation time means higher framerate.
Now, here's the rub. Collision meshes are also used by the Havok physics system, and are usually slightly larger than the object mesh itself. This means a visible object mesh can be slightly underground and so invisible, but its invisible collision mesh can be poking slightly above ground and an intersection test will detect it, so the game WILL display info for an object you can't actually see!
On PC, the trapdoor must be sufficiently below the floor that the game doesn't detect an intersection with its collision mesh. The ray intersects with the floor first, which isn't activatable. However, it's all too easy in the CS to accidentally move an object slightly when you click it. On Xbox360 (and maybe on PS2 as well), the trapdoor is probably raised slightly higher. Its invisible collision mesh is poking slightly through the floor and everything looks OK because you still can't see the trapdoor but, from a specific position, the game WILL detect it and display its info!
The playtesters and designers obviously missed the slight positioning error that crept in. Easily done because it looks OK both in-game AND in the CS, and it's hard to get into the right position. You can view collision geometry in the CS, but only for ALL objects, and the mess of red lines over everything makes it VERY difficult to spot a positioning error that leaves a collision mesh exposed when it shouldn't be.
Hope I haven't fried your brains too much.
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