How do websites get recognized as in the intranet zone for IE compatibility mode

Question

My company has thousands of clients, but about 40-50 of those are experiencing issues with IE compatibility mode which our website cannot have enabled to function. I've done my research and understand the solutions relating to document types, meta tags, and HTTP headers. A good overview of those here. For some of our clients unchecking the box that says display intranet sites in compatibility view resolves their problem however it is unclear what causes the site to be viewed as an intranet site. We have made sure that the site isn't included in the intranet zone and the client's IT admins say that it is not flagged as an intranet site through group policy (for what that's worth). My question is, are there additional ways that IE can recognize a site as a member of the intranet zone? For instance, can firewalls or network switches flag a site as an intranet site and conflict with group policy? Are there other ways this can happen? I believe I've hit a don't know what I don't know situation and it's sparking my curiosity.

Answer 1

Well it turns out what I was looking for had to do with proxy servers. In Internet Options > Security Tab > Local Intranet > Sites there is a checkbox on the pop up that says "Include All Sites That Bypass the Proxy Server." This was the culprit for the clients we were having issues with.