Housings are typically used to protect sub-assemblies in self-service equipment such as key-pad assemblies in automated teller machines (ATMs), self-service kiosks, pay-as-you-go photocopiers and the like. There is a need to secure those housings to prevent access to and tampering of the equipment within. In addition, when replacement sub-assemblies are used, for example, for maintenance or upgrade, then it is necessary to authenticate those sub-assemblies. By providing markers fixed in sub-assembly housings and sensors in the housings or self-service equipment, it is possible to overcome these problems. The markers and sensors are brought into alignment when the sub-assembly housing is fully assembled. The markers and sensors are arranged to detect when a particular physical relationship between the markers and sensors is lost or altered. For example, the markers provide spectral signatures and comprise rare earth metals such as lanthanides.