A rather frustrating behavior has been encountered in Safari 4 and 5 for Windows.  The problem is that while searching for a string in page content, the focus can suddenly be removed from the search field.  As is usually the case, no extensive troubleshooting has ben done to isolate the issue.  And it's doubtful that any will be done.  The find feature is pretty straightforward and there's no real reason that the focus should be stolen.  There are however, some factors that may come into play.

The problem seems to be encountered with pages with lots of content and search results numbering perhaps 75–100 or more.  To give it a look, here's what you try:

1. 1.Load a page with a good deal of content.
   

2. 2.Open the Find banner [Edit | Find ⌘] and start typing a string.
   

3. 3.Safari will starting matching string in the page content as the search string is entered.
   

4. 4.At some point the focus will be removed from the find field.
   

Overall, the find feature in Safari is pretty slick and so dealing with this bug may be worthwhile, but it can become quite tedious, real quick.