Embodiments of the present invention provide a detector apparatus for detecting a physical resource employed in providing a particular virtual resource in a computer network, the computer network including a plurality of physical resources each being operable to be employed in providing virtual resources and having an environment sensor outputting sensor data representing changes in an operating property of the physical resource. A detector apparatus embodying the present invention comprises a sensor data receptor operable to receive sensor data output by the environment sensors, a pattern extractor operable to extract a pattern from the received sensor data from a physical resource, and a pattern matcher, wherein the pattern matcher is operable to compare the extracted pattern with a unique pattern known to be generated by a particular virtual resource, and to detect that the physical resource is employed in providing the particular virtual resource when a match is found.