I know that episode 2 did not end the story. However, http://half-life.wikia.com/wiki/Future_of_the_Half-Life_seri... indicates that it did not contain any teasers. I don't remember the details of the ending since it has been a few years since I played it.
But in any case, leaving the story with a loose end is not the same as announcing or hyping a game. I don't see anywhere Gabe has even indicated they are working on HL3.
I don't remember the details of the ending since it has been a few years since I played it.
This is a big problem with this long dev cycles. There are a few games where I was a big fan of the previous games but when the sequels eventually came, I had lost interest because I couldn't remember the precious game's story and did not want to replay it. Of course nowadays I'd just get the rundown online or watch the ending on youtube, but unless you're a big fan, it IMHO still dampens the excitement somewhat. At least for me.
But in any case, leaving the story with a loose end is not the same as announcing or hyping a game. I don't see anywhere Gabe has even indicated they are working on HL3.