If a bag of flour were exploded in your kitchen, covering every surface with fine white powder, could you walk out of the room without carrying any trace of the flour with you. It is virtually impossible. We face the same problem designing information security systems.
Information by itself is usually static but add people and it explodes onto networks, laptops, USB drives, PDA’s and even (heaven forbid) paper. You can’t secure it until you figure out where it actually is.
This is probably the largest challenge in Information Security.