Ultimately, Google follows links. If your search form uses the "get" method so that any search results page has a URL that can be used to repeat that search, and then someone copies that URL and uses it in a link on a page which Google spiders, then Google will find that link, follow it, and "see" the result as a page on your site which it can parse and catalog (just like any other web page).
Just like the three most important things in real estate are location, location, location; the three most important things in SEO are external links, external links, external links (with content, content, content being the next most important).