I also get this when using the HTTPS everywhere plugin. This plugin has a list of sites that also have https instead of http. So I assume before the actual request is made it is already being cancelled somehow.
So for example when I go to http://stackexchange.com, in Developer I first see a request with status (terminated). This request has some headers, but only the GET, User-Agent, and Accept. No response as well.
Then there is request to https://stackexchange.com with full headers etc.
So I assume it is used for requests that aren't sent.