A system for monitoring a container comprises an RFID device interior the container and a receiving device exterior the container. The RFID device transmits messages each including unique identifying information, and the receiving device exterior the container receives and stores and/or relays messages received from the RFID device interior the container. Ones of the messages transmitted by the RFID device when the container is closed are not received by the receiving device and ones of the messages transmitted by the RFID device when the container is not closed are received and stored by the receiving device, whereby the messages stored in the receiving device provide an indication of whether the container has or has not been opened. Messages may include power level codes and be transmitted at more than one power level, and may be processed for locating and/or tracking the container.