An apparatus for detecting a moving ferromagnetic target object in a protected zone comprising: 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; 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 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; and a signal processing circuit arranged in communication with the two magnetic sensors; in which the signal processing circuit is configured to: a) process each of the first and second measurements signals to produce a respective first intermediate signal (|ΔB1|) and second intermediate signal (|ΔB2|) 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 being proportional to the absolute change in magnetic field generated by the movement of the target object; and b) process the two intermediate signals (|ΔB1|, |ΔB2|) 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; and wherein the signal processing means is arranged to indicate that the target object is in the protected zone or the non-protected zone according to the sign of the output signal. 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; 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 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; and a signal processing circuit arranged in communication with the two magnetic sensors; in which the signal processing circuit is configured to: a) process each of the first and second measurements signals to produce a respective first intermediate signal (|ΔB1|) and second intermediate signal (|ΔB2|) 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 being proportional to the absolute change in magnetic field generated by the movement of the target object; and b) process the two intermediate signals (|ΔB1|, |ΔB2|) 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; and a) process each of the first and second measurements signals to produce a respective first intermediate signal (|ΔB1|) and second intermediate signal (|ΔB2|) 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 being proportional to the absolute change in magnetic field generated by the movement of the target object; and b) process the two intermediate signals (|ΔB1|, |ΔB2|) 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; and wherein the signal processing means is arranged to indicate that the target object is in the protected zone or the non-protected zone according to the sign of the output signal.