When a thread is moved, the Staff member who moves it can choose whether or not to leave a redirect, i.e. a link to the relocated thread from its original location.
The options are No Redirect, Permanent Redirect (these are self-explanatory) or an Expiring Redirect. This is the default, and the default period is 1 hour. This means that when a thread is moved, a redirect is left in the original location pointing to the new location, just as it would be with a Permanent Redirect. However, once that period of time has elapsed, the redirect notice is automatically removed.
For example, I have moved this thread to the Chillout Room with the default settings, so in about an hour the redirect notice in General Discussion will disappear. The thread, however, will not, and will still be found in its new location in the Chillout Room.