An apparatus for locating a measurand anomaly, such as a hot-spot, along an optical waveguide is provided comprising: an optical waveguide, a light source configured to transmit pulsed light along the waveguide, and a first and second set of sensors provided along the waveguide. Each sensor is configured to reflect a portion of light propagating along the waveguide at a respective sensor wavelength corresponding to a measurand. The first set of sensors provides one or more groups of sensors configured to detect a measurand anomaly within that group. The second set comprises a plurality of sensors each separated from the adjacent sensor of that set by a distance along the waveguide greater than half the distance travelled by the light along the waveguide during the pulse duration. A plurality of sensors of the first set is provided between each adjacent sensor of the second set. The apparatus further comprises a detector configured to monitor the light reflected by the sensors, and a control system configured to control the light source and the detector to both locate at least the group containing a measurand anomaly and to monitor the measurand using the second set.