An active radio-frequency identification, RFID, tag, comprising: a power source; a radio transceiver; and a processing unit, coupled with the power source and the radio transceiver, and configured to cause the following: transmitting continuously a signal comprising an own identifier according to a predetermined schedule; receiving continuously a plurality of signals (140B, 140C, 140D) comprising a plurality of foreign identifiers (122B, 122C, 122D) transmitted by a plurality of other active RFID tags (100B, 100C, 100D) according to the predetermined schedule; measuring a received signal strength indication, RSSI, of each received signal (140B, 140C, 140D); storing each foreign identifier (122B, 122C) of a signal (140B, 140C) whose RSSI exceeds a predetermined strength threshold; detecting that a signal with a stored foreign identifier is no more received according to the predetermined schedule and marking said stored foreign identifier; characterised in that the processing unit is further configured to cause the following: if two or more stored foreign identifiers (122C, 122D) are marked, transmitting an alarm signal indicating that the active RFID tag having the own identifier has been moved. a power source; a radio transceiver; and a processing unit, coupled with the power source and the radio transceiver, and configured to cause the following: transmitting continuously a signal comprising an own identifier according to a predetermined schedule; receiving continuously a plurality of signals (140B, 140C, 140D) comprising a plurality of foreign identifiers (122B, 122C, 122D) transmitted by a plurality of other active RFID tags (100B, 100C, 100D) according to the predetermined schedule; measuring a received signal strength indication, RSSI, of each received signal (140B, 140C, 140D); storing each foreign identifier (122B, 122C) of a signal (140B, 140C) whose RSSI exceeds a predetermined strength threshold; detecting that a signal with a stored foreign identifier is no more received according to the predetermined schedule and marking said stored foreign identifier; characterised in that the processing unit is further configured to cause the following: if two or more stored foreign identifiers (122C, 122D) are marked, transmitting an alarm signal indicating that the active RFID tag having the own identifier has been moved. transmitting continuously a signal comprising an own identifier according to a predetermined schedule; receiving continuously a plurality of signals (140B, 140C, 140D) comprising a plurality of foreign identifiers (122B, 122C, 122D) transmitted by a plurality of other active RFID tags (100B, 100C, 100D) according to the predetermined schedule; measuring a received signal strength indication, RSSI, of each received signal (140B, 140C, 140D); storing each foreign identifier (122B, 122C) of a signal (140B, 140C) whose RSSI exceeds a predetermined strength threshold; detecting that a signal with a stored foreign identifier is no more received according to the predetermined schedule and marking said stored foreign identifier; characterised in that the processing unit is further configured to cause the following: if two or more stored foreign identifiers (122C, 122D) are marked, transmitting an alarm signal indicating that the active RFID tag having the own identifier has been moved.