Nigel,
Usually a thread is not deleted by a moderator unless that thread is started by a spammer and the thread only contains the one post. If a thread like that happens and another member has posted in it also, then we try to let the others who have posted there know that we are deleting the thread.
There could also possibly have been an issue with the software, and on very rare occasions - a thread might be deleted at the request of the original poster. I can only think of one even remote possibility of that happening in the last two years. Usually the thread might get locked instead.
What thread were you looking for?
Peter