Sudoku – Hidden Achievement in Pink Pointer’s Game: How to Unlock

You can see the full list of Steam achievements at the steam site.

Out of the 60 achievements there are 20 that don't have a description. 10 of them are plot related achievements and you will unlock all of them (except from the Firestorm one) if you finish the original game:

• Prisoner of War: Capture a live alien.
• The Gatekeeper: Stun an Outsider.
• X Marks the Spot: Uncover the alien base's location.
• Off My Planet: Recover the Hyperwave Beacon.
• Ride the Lightning: Build a Firestorm. (not essential for the plot)
• See All, Know All: Build the Hyperwave Relay.
• Beyond the Veil: Find a soldier with the Gift.
• And Hell's Coming With Me: Successfully assault an Overseer UFO.
• On the Shoulders of Giants: Build the Gollop Chamber.
• The Volunteer: Make contact with the Ethereal hive mind.

UPDATE:

There are 10 more that don't have a description, 5 of them are related to the slingshot DLC, those are:

• New Friend: Complete Friends in Low Places.
• Bait the Hook: Complete Confounding Light.
• The Bigger They Are: Complete Gangplank.
• Rising Dragon: Take your new ally into the final challenge.
• All Aboard: Complete Confounding Light with more than 3 turns left on the timer.

The other 5 do not appear on the xbox 360 version or the PS3 version, and have 0% completion, which means that nobody was able to get them, just as it was with the above 5 before the release of the slingshot DLC, those are:

• Disaster Averted: Complete Deluge.
• The Chase Is On: Complete Portent.
• Great Minds Think Alike: Complete Furies.
• No Sweat: Complete Deluge without activating more than 2 valves.
• Saved to Savior: Have your newest ally volunteer for the most difficult duty.

As you can see, their description, taken from here, makes no sense regarding to the game. With the history regarding the slingshot DLC they are probably related to a future DLC.

You currently miss the Freedom achievement, that you would get for "finishing" the game by getting 10,000,000 points in a round, and gaining your freedom by entering the door on the top right. This is the only achievement which is hidden until you get it.

You can see the complete list of achievements to compare.