Markchains are tiny, self-sustaining worlds of microorganisms that the ogres wear.
They can smell the presence of the essenseal in Markchains, and if they don't detect it on someone walking through their city, they know they're dealing with an intruder.
Elves and goblins can't see, smell, or taste the enzyme without reveldust.
The only elf ever who was granted a markchain was Lady Cadence, who discovered that it contained microorganisms when she walked through a cloud of reveldust.
Lady Cadence gave her markchain to help Sophie get into an ogre city.
It was first mentioned in Book 2: Exile.