"If the user chooses to render a site in Compatibility View (click Tools > Compatibility View Settings) then IE will send a User-Agent string that mimics Internet Explorer 7’s UA string:
Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.3; Trident/7.0; .NET4.0E; .NET4.0C)
By default, sites in the Intranet Zone render in Compatibility view, so this is the User-Agent string they’ll see."