Apparatus suitable for detecting a ferromagnetic object in the vicinity of a doorway, the apparatus comprising: a passive primary sensor apparatus comprising at least one first magnetic sensor, the primary sensor apparatus adapted to measure an ambient magnetic field or gradient within a localised volume of space defined by a first zone of sensitivity of the at least one magnetic sensor, and to produce a corresponding measurement signal, a secondary non-magnetic sensor apparatus comprising at least two secondary sensors (34a, 34b), each one being adapted to detect the presence of objects within a respective zone in the vicinity of the primary sensor apparatus, the zone for one of the secondary sensors being distinct from the zone of the other one of the secondary sensors, a signal processing circuit arranged in communication with the primary and secondary sensor apparatus, and a warning device operable by an output from the signal processing circuit, the warning device adapted to provide within the vicinity of primary sensor apparatus at least one of an audible and a visible warning (20, 22), wherein the signal processing circuit is configured to identify temporal variations in the measurement signal due to the movement of a ferromagnetic object within the ambient magnetic field and to correlate the identified temporal variations in the measurement signal with instances in which both sensors of the secondary, non-magnetic, sensor apparatus detect the presence of an object in their respective zones, and to provide an output indicative of the presence of a ferromagnetic object in the vicinity of the primary sensor apparatus in the presence of a correlation there-between, characterised in that; the secondary sensors (34a, 34b) are adapted to be arranged so that as a swinging door in the doorway moves from fully closed to fully open it passes through both of the zones of the secondary sensors but is only ever detected within one zone for any given position of the door. a passive primary sensor apparatus comprising at least one first magnetic sensor, the primary sensor apparatus adapted to measure an ambient magnetic field or gradient within a localised volume of space defined by a first zone of sensitivity of the at least one magnetic sensor, and to produce a corresponding measurement signal, a secondary non-magnetic sensor apparatus comprising at least two secondary sensors (34a, 34b), each one being adapted to detect the presence of objects within a respective zone in the vicinity of the primary sensor apparatus, the zone for one of the secondary sensors being distinct from the zone of the other one of the secondary sensors, a signal processing circuit arranged in communication with the primary and secondary sensor apparatus, and a warning device operable by an output from the signal processing circuit, the warning device adapted to provide within the vicinity of primary sensor apparatus at least one of an audible and a visible warning (20, 22), wherein the signal processing circuit is configured to identify temporal variations in the measurement signal due to the movement of a ferromagnetic object within the ambient magnetic field and to correlate the identified temporal variations in the measurement signal with instances in which both sensors of the secondary, non-magnetic, sensor apparatus detect the presence of an object in their respective zones, and to provide an output indicative of the presence of a ferromagnetic object in the vicinity of the primary sensor apparatus in the presence of a correlation there-between, characterised in that; the secondary sensors (34a, 34b) are adapted to be arranged so that as a swinging door in the doorway moves from fully closed to fully open it passes through both of the zones of the secondary sensors but is only ever detected within one zone for any given position of the door.