Abstract: A method and a device for detecting and localizing unwanted matter, e.g., cement deposits, internally in a drill string. A sleeve-shaped indicator body is dropped down through the drill string while the same occupies an operative position. Then, the indicator body lands either in a seat at the lower end of the drill string, or the indicator body becomes stuck within a pipe section at a higher level in the drill string. The drill string overlying the position of the indicator body may be considered free of deposits formed on the pipe section walls thereof, and these pipe sections thus may be re-used without any further examination or cleaning. If stuck at a higher level, the indicator body is subsequently localized at the surface through examination in context of disassembling the drill string. The indicator body also may comprise an indicator element that provides a detection signal or sign for its detection and localizing on the surface.