It has been interesting to read the comments here. At places I've worked I've always worked my full notice (usually months), however when other people have quit they've been locked out of their systems, and some escorted from the premises. Perhaps I come across as a naïve/nice/non-malicious. My roles have always been in some of the most sensitive positions within organisations. Often without a clear successor so perhaps the worry was about not being able to fix things if I was to become disgruntled. (Or simply not knowing if they could lock me out... as crazy as that sounds!)