Currently when you enter a search query that contains a ":" at the end (e.g. because you want to restrict your search to a certain namespace and enter that namespace with a ":" at the end) you are redirected to the page that is constructed from the search query with the start-page name added at the end instead of the search results. The easiest solution might be to prevent the redirect during the search action or maybe even during all non-show actions as this is also a problem when the action is edit (e.g. doku.php?id=playground:&do=edit is a redirect back to the show action) and only fixing this for the search action will then break the other action links.
A really simple fix is to execute the redirect in getID() only when the current action is "show", I can't imagine this change will cause any problems as the global $ID will still be the correct id, a short test seems to confirm this. I suggest to apply this fix for the spring 2013 release.
 Pull request https://github.com/splitbrain/dokuwiki/pull/197
contains my proposed fix.