A Japanese ship was sailing in the Ocean. The captain of the ship went for a bath. He removed his ring and kept it on the table near the engine.

When he came back, he found his ring missing. The captain called all five crew members who were working around that time.

1. The cook said, "I was in the fridge room getting meat for cooking."
2. The Engineer said, "I was working on the generator engine."
3. The seaman said, "I was on the mast, correcting the side of the flag which was upside down by mistake."
4. The radio officer said, "I was messaging to a company about the arrival."
5. The navigation officer said, "I was sleeping in my cabin."

The captain caught the crew member who was lying, thereby accurately identifying the thief. How did he manage to do that?