Weird fact: There was a PlayStation 2 version of the original Half-Life. Weirder fact: It contained a filing cabinet that would move to face you if you approached it.
People have posted videos of this filing cabinet for years. The exact cause of it remains unknown, but there have been some compelling lines of speculation, such as 1) it's haunted, and 2) ValveTime's recent assertion that it has to do with a goof in the way the game was coded.
— ValveTime (@ValveTime) September 20, 2017
Half-Life was ported to PS2 by Borderlands developer Gearbox, rather than by Valve. ValveTime, a site that frequently investigates longtime Valve-related mysteries, explained on Twitter yesterday that Gearbox's particular version of the game's code contains a feature called "head controller" that lets NPCs track the player's movements and move accordingly. ValveTime speculates that this feature somehow made its way into our old pal Mr Vaguely Sentient Filing Cabinet. Maybe it was an accident, or perhaps it was an Easter egg. It's tough to say.
It isn't "case closed", but it's probably as close as we're gonna get for the time being. Again, though, the filing cabinet could just be haunted.