A security screening apparatus for use in detecting ferromagnetic objects comprisesa detector apparatus which comprises a housing that supports at least one magnetic sensor arranged to produce a signal indicative of an ambient magnetic field or gradient over a zone of sensitivity which extends away from the housing, the size of the zone being large enough to contain the whole body of the person, and a signal processing circuit which receives as an input the signal from the magnetic sensor and which, in response to a change in the signal produces an alert signal. A beacon that comprises a light source is able to emit at least two visually distinct colours of light, and a control means controls the light source such that in a first condition in the absence of the alert signal the beacon emits a first colour light and in a second condition corresponding to the presence of the alert signal the beacon emits a second, different, colour of light. The light source is arranged such that the position and/or size and/or shape of the illuminated area of the light source in the first condition is different from the position and/or size and or shape of the illuminated area of the light source in the second condition.