A seal for monitoring or detecting tampering of a compartment or container, in particular a truck trailer, comprises an RFID reader 2 and RFID tag 1 which are mounted to a door 6 or opening and a frame 5 adjacent the door/opening. The tag and reader communicate with each other and a random number is generated and displayed on a display 3 provided on the reader. When the door/opening is opened such that the tag and reader are no longer within range of one another the reader displays the message 'open' . Upon closing the door a new random number is generated and appropriate monitoring of the random numbers displayed can determine if the door has been opened or otherwise tampered with. The tag transmitter may be of the active type which includes its own individual power source. It is designed to transmit at regular intervals, a validation code to the reader receiver which listens for the validation code to see if the tag is in a valid proximity to the seal.
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