Referrer URL is empty

This is actually not an issue. There might be several reasons why the referer URL would be blank. It will/may be empty when the enduser:
  • entered the site URL in browser address bar itself.
  • visited the site by a browser-maintained bookmark.
  • visited the site as first page in the window/tab.
  • switched from a https URL to a http URL.
  • switched from a https URL to a different https URL. (only if it is blocked by referrer metatag on website)
  • has security software installed (antivirus/firewall/etc) which strips the referrer from all requests.
  • is behind a proxy which strips the referrer from all requests.
  • visited the site programmatically (like, curl) without setting the referrer header (searchbots!).

source: http://stackoverflow.com/questions/6880659/in-what-cases-will-http-referer-be-empty