In a tidy house or office with a clean floor you can see immediately if someone has dropped something, a glasses case, for example. Lying on the floor, this object catches the eye, and everybody passing by will notice it. In the Space Station a number of factors make it very difficult to find something that is lost.
More at:
http://www.esa.int/export/SPECIALS/Cervantes_mission/SEMIVR7O0MD_0.html