While I’m in the early days of learning Magento, one thing I’ve found had never been corrected and always needs fixing, is the search field. Every single install I’ve done, the search field value is populated by the current URL path. While I suspect Nginx is the cause (how it handles rewrites differently to Apache), I still have to hack the theme a bit to make it behave. From memory the file to edit is/app/design/frontend//templates/catalogsearch/pmini… Something something. You have to comment out the PHP that populates the search field value.