My Best Friend Stayed at My Room and Moved Out, but There's Been a Constant Strange Smell Ever Since! the Renovation Crew Came in and Exclaimed in Alarm: "Call the Police!"

My best friend is my closest friend. She went to the city for an internship, and I lent her my apartment without hesitation. But after she finished her internship and moved out, I noticed a strange, faint smell in the apartment. I cleaned it inside and out three times, but the smell only got stronger, and the source seemed to be the bathroom. Having no other choice, I called a renovation contractor to check it. He immediately spotted the ceiling: "The color of this ceiling panel is wrong; it looks like it was added later." He climbed up the ladder, and as soon as he removed a corner, he shuddered, and his tools clattered to the floor. He even...