When doing a search in the history panel the current behaviour is to not expand the
grouped search results. So you only see that e.g. you got a result on "Monday, April 26, 2010". There is also no "+" that indicates that there is actually a result. To show it you have to double-click on the date.

When you have many results it is much work do look into every shown group. 
it would be better to automatically expand the grouped search results by
default. So it would be easier to find the search result you are looking for.