Open Web search string for the Sonar forum

You are probably correct in assuming it's a thread that no longer exists. If it can't find the specified thread, it'll back up to the first part of the URL it can find.
 
For the most part, threads are not deleted, as evidenced by the necro threads so often revived, going back at least 10 years. As an experiment, I brought up this thread from 2003. So my guess is the thread you're looking for is indeed gone. You might try searching by the thread title instead of the thread ID, if you remember what it was.

Thanks for your replies.  This is what I was looking for, but good background, too, on how the threads work.  I've been pasting links to threads in a word document for my own reference, but see now that the thread title is just as important as the number when it comes to using the google search, if not more.