A intrusion detection method is disclosed for use in a wireless local area data communications system, wherein mobile units communicate with access points, and wherein the system is arranged to locate transmitters using signals transmitted by the transmitters. A database relating authorized transmitters to location is maintained. Selected signals are detected at the access points and location data corresponding to the selected signals for use in locating a source of the signals is recorded. The source location is determined using the location data, and the source location is compared to a corresponding location in the database. An alarm is signaled if the source location is inconsistent with the corresponding database location.