Ask Conan, one episode said that it was a murder in the secret room: 100.

From the beginning, the key was there. There was a small hole in the glass of the skylight, and the prisoner threw the key in after committing the crime. After finding the body, he took the initiative to go in and broke the glass window, so which small hole disappeared, and the secret room technology was completed.

Conan just found out which glass is faulty. The jewel given to Lan by divination is the pendant on the door key. Which man recognized the key to the pendant at a glance when he entered, so Conan decided that he was the murderer.

It's not what you said. I made it clear that the murderer threw the key into the room through the small glass hole on the roof after committing the crime, but the victim gave Lan the pendant of the original key and later replaced it. Of course, no one knows what the changed pendant looks like except the murderer. Conan saw who broke the glass and went in, so he knew for sure that it was the key to the door, so he was the murderer!