I'm helping a colleague with their installation of Omeka. They are running version 1.5.1, so it's a little older than the current version. Currently if you run a search for an item or word that exists somewhere in the collection, it returns items related to your search term. However, if you run a search for something that is not in the collection you get an error message:
An item has not been set to be displayed on this theme page! Please see Omeka documentation for details.
And then it includes references to 23 places where things are going wrong, only two of which are in the theme folder. Additionally when you click on the advanced search link, it gives a similar error. I'm posting in the hopes that this is some error you've encountered before and have since resolved. Any thoughts or ideas would be appreciated.