When mods move threads, they have the option of leaving a designation saying that the thread was moved. If you click on the link and get that 'you do not have permission' message, it's very likely that it was moved to the holy Moderators' Corner. At times they leave no trail.

Sometimes threads need cleaning up and sometimes they just aren't appropriate to leave on the public forums. The thread still exists, you just can't access it anymore.

If the thread got deleted you'll just get a message that says the thread no longer exists.