HTTP Header Referer
The HTTP Header “Referer” is used maily to detect the URL from which a page is loaded. But there are times when this header is not passed. This also can be spoofed using some plug-ins. This article has more details on this. https://isc.sans.edu/forums/diary/When+does+your+browser+send+a+Referer+header+or+not/16433/
