Abstract: A tag (2) is attachable to items such as a briefcase or a laptop and communicates via Bluetooth™ (4,8) with a mobile (cell) phone (6). In use, a user carries their mobile phone on their person and the proximity of one or more of the tags is monitored (14). When a tag goes out of range then alarms (18,20) are triggered (16). The transmitter device (tag or phone) transmits radio signals with a plurality of set transmission powers (5), each corresponding to a respective set proximity, with Bluetooth power adaption turned off. The proximity is monitored by using a correlation between RSSI signal strength (10) and BER link quality (12) measurements. The proximity may be monitored by using a delay count measurement, including a measure of the time to successfully transfer data to a buffer (9). The signal strength (26), link quality (28) and delay count (30) measurements are combined into a confidence measure (32) to monitor proximity.