I also think, "Search in Topic" box should appear only if a topic has more than 1 page of messages.
Well.. I tend to think it should always stay. Otherwise it creates unnecessery confusion why sometimes it appears, and sometimes not. I guess users prefer to be comfortable and know what to expect.
The same applies to the 'Latest post' icon (s.gif). I cannot test it now but from what I noticed the icon doesn't appear if there's no reply. While logically it may make sense, I think it's not usable- because when user learned the icon is there in 99% cases, there's no need for him to wonder why in this 1% the icon is not there. If certain habit is learned, making it complicated by removing some cases may not be the best way.
At least that's my humble opinion ;).