Disappearing threads..?

[Velocd]

I have a small little annoyance on my forum I've noticed lately, that I've also noticed vBulletin.org sorta does the same.

Threads on my forum seem to automatically disappear after they get a certain date old, yet they only disappear from the forumdisplay.php showing. If you try and search for threads that might have disappeared, you will get results.

vBulletin.org does the same thing, where if you go to a specific forum and view the oldest thread, usually if you search that same forum and try to find an older one, you can.

I'm sure this is some stupid little option right under my nose in the ACP, but I can't seem to find it :dead:

Basically I don't want it to hide older threads in each forum, and just display them all with whatever pages. If you can help, that would be great.