I finally found one last year. I kept smelling something burning, looked and looked, finally found this weird little mechanical space behind a closet, found the plastic duct snaking through there, felt it, kinda warm, then when I lifted it slightly, it kinda melted/fell apart, and all the smoldering that had been going on inside burst into flame. Then it was me frantically stamping out a bunch of little lint brush fires. The client just stood there with their mouth in a little "O" shape..... I got to look smart that day.