A security device for attachment to a container having external walls such
that a transponder thereon is located externally of or adjacent to one of
said external walls so as to avoid being shielded by a metal layer within
the container (e.g. the metal layer within a disk such as a CD or DVD).
The security device is arranged to trigger an alarm signal if the
container is moved within the range of an alarm unit with the security
device attached thereto, and has a releasable locking mechanism to secure
it to the container such that release of the locking mechanism requires
the use of authorized release means. In one embodiments the transponder
is located in the vicinity of a window in a metal layer, e.g. at the
centre of the disk.