2019, November. During renovation work in the Vigadó concert hall, a mason discovers a sealed copper tube hidden behind a marble panel. Inside: a handwritten letter on yellowed paper, written in an unsteady hand. No date. No signature. “You found it. I was never sure anyone would. I hid something in this house — something the world forgot, but the house did not. It is still here, waiting. Follow my marks. Start at the face of the house.” Who wrote it? That, too, is part of the mystery.