While perusing my stats I noticed that there are several secret achievements in Runner 2. Given the quirky off-beat nature of the game, I'm curious to know if they are something I will unlock as the game progresses, or if they are of the very obscure variety. If they are part of the plot, please spoiler tag - and I'll find them when I get there. If not, I don't mind finding out now so I can get started doing whatever weird thing it is.
While I don't recall there being any secret achievements for the Steam version, there are a total of 4 secret achievements for the Xbox 360, and 1 secret trophy for the PS3.
The Xbox 360 achievements are:
Each of these is pretty self explanatory. Meaning, to get Bullseye, you need to Perfect+ a level, to get Marathoner, you need to run a total of 26.2 miles, etc.
The secret trophy for the PS3 is:
Not really sure why the PS3 version has fewer "secret" trophies, but, I don't make them! I just try and get them. Regardless, all of these are incredibly easy to earn in a single play through of the game. Just make sure you use a different character while you're dancing your way to a bullseye after 26.2 miles!
There are no "secret" achievements. Each achievement states how to get it, and it's relatively straightforward.
For the ones you are referring to, those are considered "secret" because it requires unlocking "off the beaten path" levels (the ones that are dead ends). You can pretty much get the majority of those achievements in the first level.