What causes HTTP Referer to show up as empty/blank in web activity reporting?

[Jackson_Fisher]

I pulled a web activity report today and saw that roughly half of our traffic did not have any sort of HTTP Referer. What would cause this field to show up as blank? Could it have something to do with the set up of the web page/ landing page/program that was created in marketo?

For some additional context, the majority of the blank/empty fields are related to campaigns being fulfilled by our sister team. I imagine that something is missing from their programs, that is already in use by my team. I am hoping to identify what exactly that is, so we can update their programs. 

[SanfordWhiteman]

Referrer isn't created on the Marketo side. It's supplied by the browser.

But there are numerous restrictions on when the referrer can be read by the browser at all. If the referrer is a secure SSL site but your LP domain is not secure, for example, the referring site must *explicitly* tell the browser to forward its full/partial identity -- otherwise, there will be no referrer.