Lots of visitors may arrive at your site from a clicked link, and yet their browser sends no referrer. There are many ways this can happen:

- They clicked on a link in an email (not always true. If they used some kind of web mail, the web mail server will usually be the referrer)
- The link was in a document
- The link was in Skype, GTalk, or AIM
- The link was in an app and opened a browser from the app. For example, clicking on a URL in a tweet viewed in a Twitter app
- The link originates at a secure (https) page and your page is not secure (http)
- The visitor is using IE and the link to your site was in Javascript. Javascript links to your site include those that open your site in a new browser window, or any kind of javascript redirect. Many banner links are programmed this way.
- The visitor is using IE and the link to your site is from within a Flash application (there are a lot of ways to do this in Flash so there may be exceptions)
- Your landing page redirects to another page via a 301 permanent server-side redirect
- The link was on an intranet or some other website behind a proxy or corporate gateway that was set up to strip referrers from requests
- The visitor has made changes to their browser that suppresses the referrer information

This is sometimes referred to as "dark social".