This 2008 article discusses how T-rays (terahertz radiation) "could let art historians see murals hidden beneath coats of plaster or paint in centuries-old buildings" and "could also illuminate penciled sketches under paintings on canvas without harming the artwork." Includes images. From the University of Michigan News Service. (Copyright 2007 by Librarians' Internet Index, LII Week, 4/3/08)
I wonder what you'd find under the paint of your bedroom walls? I bet they could've used something like this in Twilight.