Did you know Google uses search forms to find pages?

Googlebot tries to submit searches if your website has a search form on it. Below, you can see a screenshot of a website log. It shows that googlebot submitted a search to find pages.

The following page from the Google webmaster blog explains how Googlebot proceeds when it finds a <FORM> tag. The goal was to find pages that were invisible to the public. Now, you may understand why there are options in WordPress SEO plugins to add a “noindex” to search pages. This also explains why some webmasters can find search pages in Google search results when they perform a “site:yourdomain.com” in Google.

If you want to disable this behaviour, you can add a Disallow: /search/ in your robots.txt or you can also replace the form by an Ajax search form.