Called from RedrawRoomScene only when the viewed position has bit 6 set (inside the ducts).

Expands the deck's air-duct map into the grid buffer, redraws it with all attributes black (so the deck outline is present but invisible), builds the anonymous N/E/S/W "move to:" rows, then marks the room's anchor cell bright white and walks a cyan arm along the hidden walls toward each present duct exit — the player sees only the anchor and the crawlway arms.

View type 4 (the encounter screen) resets the alien-body animation and just rebuilds the duct move-to rows instead.