The fake - but very convincing - Left 4 Dead 3 teaser that went up overnight is definitely not made by Valve.
Out of the blue, a new YouTube video popped up that got people speculating if we're approaching the reveal of a new Left 4 Dead game. The video is very well-made, but it appeared on a new, random YouTube channel and not through official means.
This lead fans to speculate that, perhaps, Valve is running some sort of meta hype campaign to get people talking before it makes the news official. This doesn't exactly sound like Valve's way of doing things, but that didn't stop the "rumours" from running wild.
Valve has since confirmed to PC Gamer that the video is indeed fake. And, in case you're wondering, the video doesn't actually have the Left 4 Dead logo. But, right at the very last second, the zombie hand holds up three fingers before it ends.
This particular visual is very well-associated with Left 4 Dead, so you can see why it got some excited. As far as we know, Left 4 Dead 3 was in development at several points at Valve, as recently as 2017, in fact.