Bob is the chief of security at an exhibition hall. His job is to make sure that all the trade shows which happen there are safe and secure events. This weekend is a big consumer expo where many companies present their new and innovative products. There are lots of expensive prototypes out there, so Bob is wary of thieves trying to steal them. So he hired some temps as additional security guards to make sure nothing gets stolen.
It's the first day and the grand opening of the trade show. The exhibitors have built up their booths and hundreds of visitors are queueing. Bob opens the gate and the visitors pour into the hall. While he looks at the moving masses for any trouble a man approaches him.
"Hello, good sir. May I introduce myself? I am the greatest thief in the world. I have picked this trade show as my target for the next days and I plan to steal many things of great value."
"Good luck, son. Not on my watch! My guys are everywhere. They won't let you steal anything. Enjoy the trade show."
Bob goes along his daily routine, coordinating his staff and making sure everything is in order. In the evening he again goes to the exit keeping an eye out for the suspicious guy from earlier. When he leaves the building, Bob stops him.
"Excuse me, Sir, mind you if I take a look into your bag?", "Sure, I don't mind", says the thief and hands his bag to Bob. Bob looks through it and finds nothing of value except for some advertisement booklets and catalogs. "You didn't steal any of these, did you"?, "Please, Sir. You are insulting me. I am the greatest thief in the world. Do you think I would be satisfied with some worthless paper? No, the previous owners gave me these on their own accord. They even insisted I take it." Having no proof that the man had stolen anything, Bob has no choice than to let him pass.
It's the second day of the trade show. Like every morning Bob controls the entrance to make sure no accidents happen while the crowd enters the building. He is again approached by the mysterious man.
"Good morning, Sir. Yesterday was a great day. I've stolen many things of great value and today I am going to steal even more."
"I've checked you. You hadn't anything of value with you."
"Oh, Sir, you are mistaken. I had loot with me worth a fortune. Today I am going to steal even more and you won't find it either."
"We'll see about that. Enjoy the tradeshow."
Bob becomes suspicious. What if the mysterious man was indeed a thief? Bob decides to leave his daily duties to his assistant and tails the man during the day. He watches how the man goes from booth to booth, looking at many exhibits but never even touching one. As a trained security guard Bob is familiar with all the tricks pickpockets use to steal from people in a crowd, but as far as he can tell the man isn't using any of them. He starts conversations with some of the exhibitors, but never even attempts to steal anything from them.
When the man leaves the exhibition hall in the evening, Bob stops him and asks "Excuse me, Sir, may I ask you a question? You didn't steal anything today, didn't you?"
"In fact I happened to steal many things today. Some of them even more valuable than those I have stolen yesterday."
"I know. I have watched you all day and I caught you. It's over. Please come with me to the security room". This was a lie. Bob hadn't seen anything. But in his years as a security guard he learned that bluffing can often be a useful tactic to get thieves to reveal information.
"Sure, I don't mind."
In the security room Bob gives the man a thorough frisk, but doesn't find anything. Knowing that it might overstain his competences Bob asks the man to take off his clothes. The man cooperates. Bob thoroughly searches every single item of clothing but doesn't find anything either. Provoked by the man's audacity he decides to do something which will definitely get him into trouble when found out, but he does so regardless. He puts on a pair of gloves and checks all orifices of the man, but again he finds nothing.
"Do you enjoy wasting my time like this? Come on, admit it. You haven't stolen anything."
"I did. But unless you find out what I've stolen the police will never believe you."
It is the third and last day of the tradeshow. And again, the mysterious man shows up. This time Bob approaches him right away.
"I am fed up with you. I don't know what you are stealing, I don't know how, but I will find out and then I will hand you over to the police."
"I wish you good luck, Sir, but I don't think you will be successful."
Bob spends the whole day pondering how the thief could be getting his loot out of the building. Maybe he has an accomplice? Maybe he smuggles it out somehow? Or maybe the goods are still in the building and he plans to pick them up later?
Bob decides to take drastic measures. Before the tradeshow is over, he goes to the PA system and announces for everyone to hear:
"Ladies and Gentlemen, this is the chief of security speaking. May I have your attention, please. We have information that the greatest thief in the world is on this tradeshow and has stolen one or more valuables. I ask you to stay in the hall and not leave it before you were searched."
Security guards are stationed at all exits of the building. Every single person who leaves the hall are searched thoroughly, but to no avail. After everyone left, the guards search every trash can, every box and every container for stolen goods. Any cavities in walls, the ventilation system, the suspended ceiling, floor shafts, everything in the building where stolen items could be hidden is searched. But the guards don't find anything.
A few weeks after the tradeshow is over, Bob goes to a bar. There he meets the man again. He is wearing a very expensive suit, lots of fancy jewlery and is drinking the most expensive champaign in the bar. Bob decides to start a conversation.
"You seem to be quite rich, aren't you?"
"Newly rich. I just sold some of the things I have stolen at your trade show. My clients paid me thousands of dollars for it."
"Please, Sir, I need to know. I swear I will not tell on you. What is it you were stealing?"
"I was stealing █████"