Valve has filed a trademark for Half-Life 3, almost a decade after the release of Half-Life 2.
Update: The filing has since been removed and is thought to be an elaborate hoax.
Valve is yet to make any official announcements, but the listing uncovered on the official European Registration Office website - filed on September 29 - would suggest that the developer could be preparing to finally break its silence on the much-rumoured sequel.
Half-Life 3 has been the subject of rampant rumour and speculation for years, while Valve has remained resiliently tight-lipped.
Artwork that was allegedly leaked from Valve's secret project emerged online last year showing character Alyx Vance in a winter setting, but those images remained unverified. The title also appeared on the official Gamescom 2012 website in a list of games that were to proposed to be revealed at the German expo that year. Valve quickly debunked the mistake.
Valve boss Gabe Newell confirmed in November 2012 that the studio was working on Source 2 engine - the follow-up to the Source engine tech which debuted with Half-Life 2 in 2004.
A former Valve developer's online CV later suggested that work on the new engine had begun in 2010, shortly after the release of Half-Life 2: Episode 2.