In UBB.threads, when you move a thread, no copy is left behind.
I'm working on a hack (not actively at the moment) to leave a "stub" which points to the moved thread.
I suppose a distinctive string could be placed in the stub, such as "<!--moved ########-->", where ######## is the timestamp of when the move took place. Then it should be fairly easy, possibly by changing doexpire.php, to get rid of the stubs.