If the number of monthly searches is higher than expected, there may be several reasons:
You are getting more traffic.
An Adwords campaign with a direct link to the search results page.
A newsletter in which you link directly to search results.
Your site may be searched by automatic search engine bots. These are usually recognised by the Findologic bot detection and are not billed. You can find out more about search engine bots in the FAQ article: How can I avoid Google crawling all search results and causing an unnecessary number of queries?
It is possible that calling up an invalid shop URL (404 error page) in your shop will automatically redirect you to the Findologic search.