According to determined intenet sleuths, Team Fortress 2 is being invaded by UFOs. Datamining the Team Fortress 2 update that rolled out earlier today, a user on the Facepunch forums discovered a bunch of new assets listed in the code. Words like 'invasion' and 'saucer' feature prominently, as does references to posters that feature these.
Then there are other TF2 players who have actually spotted UFOs by, you know, playing the game. Another user on the Facepunch forums spotted one of the aforementioned posters in the 2fort map, and YouTube user waterandroid even made a video of a flying saucer appearing in the skies above Dustbowl, as below:
Someone on Reddit has collated all of the evidence so far, and it's pretty compelling. I'd go so far as to say it's conclusive. The UFO Invasion update has been rumoured for a while after all, with this very unsubtle video leaking back in July. Oh, and Halloween is coming next month, so I wouldn't be surprised if all these sightings culminate in something around then.