The Preferred Material: Borrowing directly from video games, here, where ‘the bits you can mess with’ are often in a slightly different color… It’s possible for a given species of foe, or culture, to add to or modify locations primarily using some preferred material or materials. If the group looks down a hallway and sees that little obsidian spikes with gold bits have been pinned to the walls, this might tip them off that the hallway has been modified by the Shalaut, that terrible race of lizard-things.
Lots more where that came from, discussing not just inanimate objects but also characters and bits of information. I love the Amagi Games is shaking out to be a place to go for useful advice on issues that GMs routinely face in RPGs.
UPDATE: And be sure to check out this little interview with Levi at At Will.