I was looking over some Skyrim videos, and it was mentioned that shadow marks were symbols that directed thieves in their thievery. Whether something was dangerous, there was good loot inside, etc. there would be these shadowmarks on very inconspicuous places that those of the thieves guild would understand and follow. I was thinking they could be visible as a passive ability in the legerdemain or Thieves Guild tree, or, placed in the world in a future patch without spending a point for it. The idea occurred to me when I broke into a house and there wasn't anything in the house worth breaking into. :P