Who Stole the Necklace?

Who Stole the Necklace?

On behalf of his client, detective Kronin dropped in a nightclub at a railway station. He needed to question the visitors of the place about an incident that happened to his client a few days before.

Gathered at a small round table close to the bar-counter there was rather a mixed company. Having interviewed everyone present, detective Kronin found that the club staff with the only exception of the barman consisted of amateurs. For instance, the dancer with her dog regularly visited the club only out of her habit to idle away her time. The two waiters were fellow students at the University of Economics. The club's duty manager came to Moscow from Tambov, where she had worked as a cutter with a private company making overcoats. The guard said that he left the Russian Air Force, having served five years as a navigating officer under contract.

After Kronin's interlocutors had told a little about themselves, the DJ, at the sign from the barman, put the music on full volume and the clubroom was plunged into pitch darkness. The guard stood up, motioned the DJ to turn the volume down and commanded by radio that his colleague immediately turn the lights on. In a moment, the table was floodlit with soft light again.

All of a sudden, the woman with a dog, who was sitting with her back to the bar-counter, clutched her head and screamed: “My diamond necklace! It was stolen when someone deliberately switched off the light!”

- Who could do this? The club visitors watched each other with suspicion and hostility, - Which one was the thief?
- “Do not worry, please,” Kronin’s voice interfered. “I have long known who of those present reached out to take the other person’s stuff.”

Question

How did detective Kronin manage to notice the theft of the necklace?