Fuga: Melodies of Steel

Actually, it opens up in Chapter 3. You'll get a tutorial on the observation room so you don't have to hunt around. After you get the tutorial in Chapter 3, then you have to make sure you always go up there during intermissions.

Using the observation room doesn't cost any AP.

Be sure to keep clicking on the "Observe" until it tells you get the "There doesn't seem to be anything more to examine." message. For instance, the first time you use it in the tutorial, you can use it twice more for 3 total observations.

EDIT: It looks like this might be wrong? I do seem to be missing two locations before the Chapter 3 ones I observed currently. Will report back.

EDIT EDIT: From Chapter 3 until the end of the game there are EXACTLY 30 observations to be made so you can get it on the first playthrough if you started doing the observations in Chapter 3.

Just to confirm you can get this on your second playthrough on NG+ by just visiting the ones you missed. I didnt notice I had to do that until chapter 3 and got the cheevo after redoing chapt 1 and 2 on NG+