Apparatus for protecting an entrance to a protected area, the apparatus comprising: at least one magnetic sensor 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 non-magnetic sensor means adapted to detect the presence of objects within a primary detection zone in the vicinity of the magnetic sensor apparatus, characterized in that it further comprises an RF receiver which is adapted to receive an identity signal from a RF transducer when the RF transducer is within a detection zone, a memory which stores one or more identity codes corresponding to respective identities of RF transducers, a signal processing circuit arranged in communication with the magnetic sensor apparatus, the first non-magnetic sensor apparatus and the RF receiver, and a warning device (20; 22; 24) operable by an output signal from the signal processing circuit, the warning device (20; 22; 24) adapted to provide within the vicinity of primary sensor apparatus at least one of an audible and a visible alarm, and further in which: 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 the non-magnetic sensor means detects the presence of an object in its detection zone, and in which the signal processing circuit is arranged to cause the alarm (20; 22; 24) to operate in the event that the correlation is indicative of the presence of a ferromagnetic object in the primary detection zone but to modify the operation of the alarm (20; 22; 24) in the additional event that the RF receiver detects the RF transducer which has an identity that is stored in the memory, wherein the apparatus is adapted to suppress the alarm (20; 22; 24) for a defined period of time sufficient to allow a person carrying the RF transducer to move through the entrance or entering and then leaving the detection zone of the first sensor apparatus in which the period of time is varied according to the identity of the RF transducer that is detected. at least one magnetic sensor 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 non-magnetic sensor means adapted to detect the presence of objects within a primary detection zone in the vicinity of the magnetic sensor apparatus, characterized in that it further comprises an RF receiver which is adapted to receive an identity signal from a RF transducer when the RF transducer is within a detection zone, a memory which stores one or more identity codes corresponding to respective identities of RF transducers, a signal processing circuit arranged in communication with the magnetic sensor apparatus, the first non-magnetic sensor apparatus and the RF receiver, and a warning device (20; 22; 24) operable by an output signal from the signal processing circuit, the warning device (20; 22; 24) adapted to provide within the vicinity of primary sensor apparatus at least one of an audible and a visible alarm, and further in which: 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 the non-magnetic sensor means detects the presence of an object in its detection zone, and in which the signal processing circuit is arranged to cause the alarm (20; 22; 24) to operate in the event that the correlation is indicative of the presence of a ferromagnetic object in the primary detection zone but to modify the operation of the alarm (20; 22; 24) in the additional event that the RF receiver detects the RF transducer which has an identity that is stored in the memory, wherein the apparatus is adapted to suppress the alarm (20; 22; 24) for a defined period of time sufficient to allow a person carrying the RF transducer to move through the entrance or entering and then leaving the detection zone of the first sensor apparatus in which the period of time is varied according to the identity of the RF transducer that is detected. 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 the non-magnetic sensor means detects the presence of an object in its detection zone, and in which the signal processing circuit is arranged to cause the alarm (20; 22; 24) to operate in the event that the correlation is indicative of the presence of a ferromagnetic object in the primary detection zone but to modify the operation of the alarm (20; 22; 24) in the additional event that the RF receiver detects the RF transducer which has an identity that is stored in the memory, wherein the apparatus is adapted to suppress the alarm (20; 22; 24) for a defined period of time sufficient to allow a person carrying the RF transducer to move through the entrance or entering and then leaving the detection zone of the first sensor apparatus in which the period of time is varied according to the identity of the RF transducer that is detected.