the tag device being associated, in use, with the entity or object and comprising a radio receiver programmed to scan a predetermined radio band at different times as the entity or object traverses said path of travel, and to capture at a current location of the entity or object at each said time a frequency signature associated with said current location, said frequency signature representing the extant frequency spectrum expressed as signal strength versus frequency within said predetermined frequency band; storage unit configured to store the frequency signature associated with each said current location for later retrieval; and an output for outputting a resulting plurality of frequency signatures stored in said storage unit,

the retrieval unit comprising a computer programmed to communicate with said output of the tag device, to retrieve a plurality of frequency signatures stored in said storage unit, to compare said frequency signatures with known antenna transmission data and geographical location data to determine the location of the tag device at the time when each stored frequency signature was captured, to derive the path of travel from the successive geographical locations, and to provide a trace of said path of travel.