The highlighting appearance in Kate's search and replace function has been modified. Dominik Haumann, one of the Kate developers, changed this behavior on several user requests, who were unable to find out that pressing Escape was needed to remove these highlightings.
In the commit message he refers to the change as "a truly sad day", since he personally like the feature quite a lot. The commit message explains the new behavior in detail:
Up to KDE SC 4.9, the search & replace highlights stay, even if you close the search & replace bar. You had to press Escape to manually remove them. From KDE SC 4.10 on, there will be a notification bar on the bottom of the view that tells the user the amount of search & replace matches.
This bar stay visible for about 6 seconds, then it hides itself automatically. And then, i.e. after these 6 seconds, the search & replace highlights are also removed.
I wrote "this is a sad day", since I personally liked the previous behavior a lot. But there seemed to be quite some users who never figured out that Escape was needed to remove these highlightings :)