This is a Burp Suite extension that attempts to identify any unusual HTTP headers on requests or responses. Occasionally I come across web applications that have a custom header but these often go unnoticed because of the general background noise of other headers going on. I have used other extentions in the past and they all appear to come with a host of other features or limited lists of recognised headers and so inevitably I end up ignoring any issues they raise.
It does this by raising an issue for any header that is not on its list of known headers. I think I have caught most of them, but, if there are others you spot please raise an issue and give me some details. This BApp also has a UI element so that you can add headers during an assessment. For example, you may have an unusual header, but upon inspection it is a benign header that you aren't interested in - in which case you can prevent it from being flagged again.
This extention is designed to rarely ever trigger an alert and therefore should be investigated when it does.