These pop up fairly often, so I figured it might be a good idea to have a central repository of unannounced GOG sale promos that people can post in as these "ninja discounts" are noticed.

[NEW CLARIFYING EDIT:]
For the purposes of this thread, "unannounced" means that there has been no front page news post about the discount or sale (and it wasn't mentioned offhandedly in a different news post) -- and having a "tile" in the top section of the front page linking to a promo page or individual product page doesn't itself count as an announcement (unless the game is somewhere in the main game tile "mosaic"-thing that they do on the front page during bigger sales). A mention in a forum thread (even by a GOG staff member) would generally not count as an announcement, either, unless perhaps it's a topic created by a blue, and the mention is in the first post.
This topic is only concerned with limited-time discounts on GOG itself; permanent price changes don't belong here (there's another thread for that), nor do discounts on other sites where a GOG code is included with purchase (there probably could be another thread for that).
[END OF NEW EDIT]

Currently, there are two games (with 4 store pages) that are on sale, and which seemingly have nothing to do with any other announced release/weekend/pre-order/[whatever] promos:

Grandia II Anniversary Edition (-50%) (GungHo Online Entertainment)
République (-50%) (GungHo America)
République Deluxe Edition (-50%) (GungHo America)
République Deluxe Edition Upgrade (-50%) (GungHo America)

...So, it seems to be a "50% off all GungHo titles" promo.
(OLD) EDIT: Added catalog link.

Great idea!

If you keep maintaining this thread I'd suggest to add links to the game's names for convenience: There you go. ^_^

Oh, I don't know if I'll go full Sinistar and keep the OP fully updated with whatever's currently on ("secret") sale; I was envisioning something more like the "Upcoming list" thread, where one can just scan the last several posts or so and see what's current. But we'll see.

Good idea about the page links, though. In this case, actually, a filtered catalog link would work just as well. :)

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It's fairly boring, there's a guy that keeps talking to you using text to speech which really annoyed me.
The console version supposedly has proper controls instead of being a point and click but it hasn't been implemented into the pc version yet.

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Yeah, that works too. :)

The In Dev title <i><span class="bold">Kim</span></i> is 10% off right now.

(•_•) / ( •_•)>⌐■-■ / (⌐■_■)

Jotun: Valhalla Edition is 50% off. Didn't see any annoucement for it.

The game also got a content update that introduced a New Game+ mode. There was also no announcement about it. I wonder why some games get news post for updates and some don't. Do the developers or publishers pay for it?

Bit.Trip Runner and its sequel (but not any of the other Bit.Trip games, nor any of the soundtracks) are currently 50% off.

Firewatch 33% off now

It's a probably a bit late in the sale to be of much use, but here's a (hopefully comprehensive) list of discounted games & DLC related to discounted games advertised individually on the front page:
The Collector's Edition of Divinity: Original Sin - Enhanced Edition is 50% off;
Dying Light's [url=https://www.gog.com/game/dying_light_harran_ranger_bundle]DLC bundles are all 50% off;
Hard West's Collector's Edition is 40% off, as are the Collector's Pack upgrade DLC for the base game, and the Scars of Freedom expansion;
The Champion Edition & Royal Edition of Pillars of Eternity are each 60% off, and The White March expansions (Part I, Part II, & the Expansion Pass which includes both) are all 40% off.

Also, seemingly unrelated to anything else, the following are also currently discounted:
Man o' War: Corsair is currently 10% off;
The Masterplan is currently 75% off.

<span class="bold"><i>Sheltered</i></span> is currently discounted by 50%. Don't ask me why it has its own promo page, though.