A method for monitoring a container having an RFID device interior comprises periodically transmitting messages including unique identifying information and receiving messages transmitted by the RFID device interior the container by a receiving device exterior the container. Some messages transmitted when the container is closed are not received by the receiving device, and some messages transmitted when the container is not closed are received. The method comprises storing and/or relaying the received messages and determining from the number of messages received from the RFID device that are stored and/or relayed whether the container has or has not been opened. The method may also comprise transmitting the messages at two or more power levels and selecting messages that are at the lowest power level that produces at least two corresponding stored and/or relayed messages for determining the location of the RFID device.