"Our approach has been, 'It's as easy to catch students who are plagiarizing as it is to plagiarize,' " Trinkle said. [from article by Piet Levy in Post-Tribune.]
IPBiz notes that Turnitin builds its own database by compiling all student submissions, not all of which will appear on the internet. Further, things like "burning my pajamas at age 8" will appear off the internet (in college application submissions) before they appear on the internet (as evidence that college applications themselves are being plagiarized). Further, the material Katie Couric plagiarized was from the Wall Street Journal, text for which is generally NOT on the internet.
A different issue is faulty indexing by Google. IPBiz has been discussing the disappearance of Vai Sikahema's (in)famous "Rutgers is Wrong" piece for a long time. On May 15, 2007, there are four hits for +"Vai Sikahema" +"Rutgers is Wrong". The point is things can be on the internet, but you can't find them with Google. [Of Rutgers, don't forget the video on plagiarism done by Rutgers University's Paul Robeson Library.]
And, don't forget that the Harvard Business Review said in 2004: Plagiarize with Pride!