This Facebook Page has artwork that
fan site ValveTime believes is from
Half-Life 2: Episode 3 circa 2008, saying: "These fantastic images have been sent in by an anonymous visitor to the site from an unknown source. Despite that, we have no reason to believe they aren't real. One could argue that perhaps they are just generic Half-Life 2 concept art of concepts that never made it into the final game. But there's plenty of evidence here to suggest they are post Episode 2."
Computer and Video Games says they've reached out to Valve for comment.