Steam lists the game as having 60 achievements, but only ~38 actually show up in the achievement list to start with.
What "secret" achievements are there, and what are the requirements to unlock them?
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:
There are 10 more that don't have a description, 5 of them are related to the slingshot DLC, those are:
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:
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.
Need to get more info on this as I've got 33 of 60. And can see some that haven't been achieved, but not all of them.
Here's the one's I know: