It would clearly tell him if it had been used in his absence - when the door is opened it will fall to the floor, most people probably do not pick it up from the floor, and certainly wouldn't replace it. How this benefits him, beyond some kind of fetish, I am not sure. Perhaps he only likes to use toilets he knows no one else has used and 'primes' several around the building every morning to guarantee he has a supply of un-violated cubicles for his use. Of course this presumes they are cleaned every night, or at least that there are enough cubicles to plausably get him through between cleanings. In old spy novels the protagonist would often stick a single hair, using a bit of saliva, between the door and door frame of their hotel room when they go out for a bit of espionage. If the hair has fallen they would know if someone had been in their room. Perhaps he is a rather clumsy John Le Carre fan.