An apparatus for detecting a moving ferromagnetic target object in a protected area comprises a first magnetic sensor adapted to measure a magnetic field within a first localised volume of space defined by a first zone of sensitivity of the first magnetic sensor and to produce a corresponding first measurement signal, and a second magnetic sensor adapted to measure a magnetic field within a second localised volume of space defined by a second zone of sensitivity of the second magnetic sensor and to produce a corresponding second measurement signal. The two magnetic sensors are separated spatially by a baseline such that the overlap between the two zones of sensitivity defines both a protected zone and a non-protected zone. A signal processing circuit is arranged in communication with the two magnetic sensors and is configured to process each of the first and second measurements signals to produce a respective first intermediate signal and a second intermediate signal which are each independent of the value of any non-changing background magnetic field that may be present in each zone of sensitivity, the value of the first and second intermediate signals thereby being dependent on the magnetic field associated with a target object moving in each zone of sensitivity. The first and second intermediate signals are proportional to the absolute change in magnetic field generated by the movement of the target object. The signal processing circuit also processes the two intermediate signals to provide an output signal that has a value that is indicative of whether the target object that caused the magnetic field is in the protected zone or in the non-protected zone.